More than Love at First Sight, more than simply two Soulmates destined to be together, the Red String of Fate is some perceivable clue that identifies your destined True Love.
The trope namer is an East Asian belief originating from Chinese legend called the Red String of Fate (akai ito or unmei no akai ito). The concept is that two people who are destined to be together are attached by an invisible red string bound from a male's thumb to a female's pinky finger (though nowadays it's become more common to show both parties attached at the pinky).
But the Red String of Fate need not be a literal red string. It could be a timer counting down to alert you at the exact moment that you meet the perfect person for you. It could even be an unusually meaningful birthmark showing your destined’s name in their own handwriting. But whether magic or Magitek, whether you’re born with it or have to have it installed, whether it actively pulls you together or serves only as a passive identifier, the match it suggests for you is your True Love Because Destiny Says So.
Warning: Choking Hazard. May be related to the Pinky Swear. Usually involved in Reincarnation Romance. See also Fate Drives Us Together and Forgotten First Meeting. Compare Love Before First Sight and Mindlink Mates.
In one episode of Ranma ½, Shampoo attempts to use an actual magical red string to bind Ranma to her romantically.
Another variation occurs in a late manga story revolving around the Tanabata, where Ranma and Akane will supposedly be destined to marry if two leaves with their names on them remain intact and bound together, but they will never be if either one is destroyed. Cue Ranma having to go through hell, including Tatewaki Kuno suddenly coming out of nowhere with a steel-bladed katana, to ensure that the leaves aren't destroyed or tied to the leaves of other people.
In the song "November Rain" from one of the albums (which is basically an angsty subversion of the Umbrella of Togetherness trope), the lyrics strongly imply that Ranma and Akane are bound together by the Red String of Fate, even though it is currently, as Ranma puts it, "loose."
Toward the end of the Urusei Yatsura movie "Remember My Love", notorious lech Ataru is shown to have multiple red strings attached to every one of his fingers. It's implied that he might like to be faithful to Lum, but...
In episode 5 of Excel Saga, "The Interesting Giant Tower", Watanabe is surprised (and annoyed) to find his roommates applying for jobs at the same office building he's gone to, and Iwata quips "We must be joined together by the invisible red string of destiny!" Watanabe flips and hits him, declaring, "The red string is for boys and girls only!"
In the same episode, Watanabe finds that Hyatt, his love interest, is at the same place selling refreshments. She gives him a drink for free, and as she walks away, Watanabe muses, "...the red string!" Sumiyoshi looks at the drink and remarks, "It's green."
Spiritual Successor OVA Puni Puni Poemi parodies this with Poemi, who's attached to her friend Futaba. At that moment, Futaba and her sisters are being taken hostage by incredibly horny aliens, and as they get dragged away in a net, Poemi is forcibly dragged to them herself. Poemi then follows the red string tied to her finger to find her friend.
A Red String appears in a flashback of Juri's recollections of her time spent with Shiori and an unnamed male character in Revolutionary Girl Utena.
In an episode of Chou Kuse Ni Narisou, a gay teacher who mistakenly develops a crush on Nagisa suggests to her that there's a Red String of Fate connecting them. Nagisa can actually see the string, and yells at the teacher to cut it out....
A Red String is used as an unseen force in Ai Yori Aoshi that binds Aoi and Kaoru together. It was part of the premise that their relationship is so unshakable that neither Kaoru's Harem nor Aoi's old fashioned and powerful family can pull them apart. It was featured in the opening sequence and referred to as Enishi (The bonds that tie) in a conversation between Aoi and Kaoru in the second season.
Also, when Mayu first sees Kaoru again, she talks about them being bound by the red string of fate (she's been nursing a crush since their first meeting years earlier). Alas, The Masquerade prevents Kaoru from telling her his string is already tied.
In Nana, Reira writes a song about her seedy relationship with Shin by describing it as taking the red string off someone's finger.
Tachikawa Megumi's one-shot manga Music Box of Spring used the Red String concept; in the end, the characters' hands are cut and bleeding, and the main character's perspective shifts, seeing the blood as the red string.
Mamotte Shugogetten has an episode with mystic threads that bind the fingers of the first two people to pick it up. Although they don't behave much like the threads of the legend, the reference is clear, as the first one to be used is red.
In the manga's 139.5 omake of Gakuen Alice, the hands of baby Mikan and Natsume are seen as connected by a (likely red) string.
Detective Conan: The Time-Bombed Skyscraper concludes with Ran selecting which color wire on a bomb to cut; the bomber had designed the bomb to go off if the red wire were cut, having overheard that red was her favorite color, but Ran couldn't bring herself to cut it, seeing it as the red string of fate between her and Shinichi.
Reversed in Noir, as Mireille (a Corsican) muses on the connection between herself and Kirika.
Yu Yu Hakusho: Kuwabara claimed that the way he found his love interest, Yukina, was simply by following the red string tied to their fingers. The anime provides a visual; in reality, he probably just sensed her spirit energy.
In an episode of the "alien arc" of Sailor Moon (R) Usagi states to her love rival En/An/Ann (urgh, transliteration problems) alias Natsumi Ginga that she is connected to Mamoru by the red string of destiny. Which appears on screen, only to have Natsumi cut it.
Kannazuki no Miko's opening features the two heroines tied by a red string. It appears in the show itself in the form of Himeko's bloodied bandage which twirl around the arms of both females as the gods take Chikane away in the last episode.
The opening of Potemayo has Mikan wearing a literal red string that supposedly attaches to Sunao, and she makes it clear during the series that she believes she and Sunao are "tied" to each other.
In Kimagure Orange Road, as Yukari sings Like a salvia flower onstage she takes a long red ribbon and playfully wraps a part of it around Madoka, throwing the other extreme to Kyouske. Here is the video
In the first ending theme of the anime adaptation of Kekkaishi there is a line which roughly translates to "there is a red string which connects two people", in apparent reference to Yoshimori's feelings for Tokine.
Mononoke contains a variation involving a red cloth between an unborn child and their parents. A much more traditional example appears in the OP, with a red string tied to the pinky finger.
I could see the red stings connecting everyone to you... I was a blue thread.
In the manga Wish, when the main character Shuichiro confesses to his adoptive mother that he loves her, she tells him that she is not the one meant for him and that he needs to find the person on the other end of the string on his finger.
Essentially the basis of Bound Beauty, only that the characters can see the other strings of fate (White, Blue, Yellow, and Black, in addition to Red). Leads to some very surreal battle scenes.
This becomes a plot-point in Mahou Sensei Negima!'s summer OVA when Yue (an Inept Mage at the time) uses this on Nodoka and Negi, creating a physical red string tying the two together by their pinkies for a day.
The trope is name-dropped in InuYasha when Kikyou tells Inuyasha that the red string of fate, once cut, can't be restored.
The final shot in the first ED sequence for InuYasha: The Final Act shows a literal red string tied to Inuyasha and Kagome's fingers.
While not actually shown in the anime, one of the DVD covers for Happy Lesson has each one of the male lead's fingers tied with a red string and all the strings being connected to all his mothers' pinkies. He is not amused.
The red string of fate has a much darker purpose in Hell Girl, where each client of Enma Ai seals the contract with her by untying a red string. This sends the object of their vengeance to hell — and damns the client to go there too, after death. So it's still a destiny bond, but a very different kind...
The end theme of Tsuyokiss shows each of the girls naked (it's a fanservice show) with a red string tied to one finger.
Parodied in Yuria 100 Shiki. The titular Sexbot fantasizes about one of these on her finger . . . connecting to Shunsuke's crotch. (The narration remarks that all of her fantasies end up there eventually.)
In Pokémon Special, it happened to Red and Yellow twice. Of course, said string (a Caterpie's String Shot and Yellow's fishing line) wasn't actually red...but yes, pinkies and shipping symbolism were still there.
In Nabari no Ou, Miharu is shown with a red string on his finger after erasing Yoite.
Used literally in Koi Cupid; red strings are seen connecting people together. If necessary, the strings can be cut with magic scissors.
In Naruto, we have a literal example. The titular character meets the spirit of his mother, who tells him that the reason she fell in love with his father was because, during an incident when she was kidnapped, he was the only rescuer who noticed that she had been leaving a trail of red hair. She commented that her red hair was her "personal red thread of fate", leading her to her soulmate.
At the end of Gundam 00's first season, Graham Akre claims that he and protagonist Setsuna F. Seiei are connected by this — the fate to face each other in battle. Though he's a Westerner, Graham is a massive Japanese culture Otaku and would likely understand the reference.
Shibariya Komachi is pretty much based on this trope, although it uses 5 different colours of thread that connect a variety of things. Protagonist Chiyako can initially see other people's red strings and takes advantage of this to make money, but ends up with the white string that binds body and soul.
In A Certain Magical Index, Index speculates that the reason why nearly any girl who meets Touma falls in love with him is that his Anti-Magic right hand, Imagine Breaker, negates the Red String of Fate for other people.
In Bleach, specifically the movie Fade To Black, Ichigo and Rukia's red spirit threads are linked to one another, allowing Ichigo to find Rukia when she went missing during the events of the movie.
A flashback reveals that Urahara connected Isshin and Masaki together with a red spirit thread so that Isshin's influence would prevent her from Hollowfying.
In the Naruto fanfic "One Small Step", a young Naruto can't find a partner to play Cat's Cradle - a game that involves using string to create complex patterns between the fingers - which could help him learn the handsigns necessary to work ninjutsu. After searching for someone to play the game with him, he finds Hinata and asks for her help. The two end up tying each other's hands together (with red string, naturally), which leads to them becoming childhood friends and starting to develop feelings for each other.
Quite a few Axis Powers Hetalia fanarts of America and Canada show a red string attached around their hands. Could be to represent their relationship which in real life is one of the closest and most productive international relationships in the world. Or it's to play up the Ho Yay.
The red string of fate has also been referenced in quite a few fanworks that pair Japan with someone. It's most common in Japan/China fanworks, probably because of the concept's origins in China and the tendency for the pairing to be portrayed as a tragic one, but appears in some Japan/Taiwan and Japan/Greece fanworks too.
In the Sherlock fic "With Your Crooked Heart”, most people (except those supposedly incapable of love) have words on their forearm which appear as if by magic at some point in their lives, usually during childhood. The words are in the handwriting of your True Love and may be a name, address, or cryptic clue like a poem or obscure reference. When the clue is obvious, like an address, you can hurry fate along by turning up on their doorstep, but if its as vague as a line of poetry, you may have to get to know your True Love before the clue begins to make sense.
In the TiMER-inspired Sherlock fic “Blue Veins”, everyone has a magical configuration of veins near the surface on the inside of the wrist that count down to the moment you meet your True Love.
In the Sherlock fic “Put a Smile in Your Name”, everyone is born with the first name of their True Love in their True Love’s handwriting on their palm. Databases of this information exist and, for a fee, matchmaking companies will check for any matches between your name and the name on your palm.
In the Sherlock fic "Finding John”, everyone is born with the first name of their True Love like a tattoo on the inside of one finger. With only a limited number of first names, it’s usually not enough information (unless you’re The British Government) to lead you to a person but once you meet your True Love, they’ll have your name too and the particular color and shade of your names will match.
This trope is defied in the Sherlock fic, "To the Limits of Your Choice" when Sherlock chooses not to care, or perhaps not to accept the consequences of acknowledging the person symbolized by the soulmate mark on his wrist. When John finds his soulmate, Sherlock asks John to choose between camaraderie with him and romance with her and John also defies this trope, choosing to continue adventuring with Sherlock.
In Kamen Rider The First, a partially-suicidal Haruhiko runs away from the hospital to a field where he can grieve alone, only to realize that a red string had somehow caught on his clothes. Behind him walks up Miyoko, his love interest, and the string was a thread from the red sweater she was wearing which had caught and began unraveling as he ran. She directly quotes the concept, and he proceeds to hug her for all he's worth. Unfortunately, since they were both terminally ill, things don't turn outvery well.
In The Secret Of NIMH, in what may or may not be an intentional usage: when Mrs. Brisby first meets Jeremy, he is tangled in red string which he is retrieving to build a "love nest" for his future Ms. Right
The end of the film has Jeremy and his love interest flying and holding the two ends of the string
In TiMER, Magitek timers installed on the wrist by a friendly Mega Corp countdown to the day you’ll meet your True Love and beep when you finally set eyes on each other. Angst Alert: you could get your timer and find out you have decades to wait until you’ll meet Mr or Ms Right or it could remain blank because your True Love is a Luddite without a timer of their own.
In Paulo Coelho's Brida, the titular girl is informed that Witches can recognize who their destined soulmate is because they can see a special twinkle in the eyes, while Mages do so by seeing a star over the shoulder of their destined. Brida, who becomes an aspiring Witch, recognizes the twinkle in her actual boyfriend's eyes and is pleased; but the Mage she initially consulted has seen the star over Brida's shoulder, and is conflicted. This is carried to a long scene where the Mage finally decides to confess his visions and feelings and use the star to try to find Brida in a crowd... and then he finds Brida's boyfriend, who also has the damn star over his shoulder. This being a No Bisexuals setting, the Mage steps out and lets the happy couple be.
In the novel "On a Pale Horse", Zane is offered a Lovestone, a magic stone which will lead him to his true love. He borrows the stone and follows its glow. The good news is he finds her. The bad news the proprietor of the store bought the encounter, in exchange for the Wealthstone, so HE gets the lovely and extremely wealthy girl. The results of this encounter start the main plot.
In Rumo and His Miraculous Adventures, the 'Silver Thread' is something that wolpertings can actually perceive through smell, and it leads to their soulmate. Unfortunately for the protagonist, he's also hopelessly uninformed about relationships, and didn't even know that girls existed until he came to Wolperting, meaning it isn't all that much use.
In L.J. Smith's Night World series, soulmates who are destined to be together often describe a silver thread connecting the two of them together.
Also in L.J. Smith's Vampire Diaries, Elena remembers stories of "the souls of true lovers" being connected by "a silver string from heart to heart or a red cord from pinky to pinky", but it's the first one that she's able to find (and follow). Stefan and Elena are bound together by the silver cord from soul to soul, implying that they are destined soul mates.
Continuing with L.J. Smith, in Secret Circle, Adam and Cassie are bound together by a silver cord, like Stefan and Elena from Vampire Diaries.
In Jane Eyre, Mr. Rochester has this to say: "...it is as if I had a string somewhere under my left ribs, tightly and inextricably knotted to a similar string situated in the corresponding quarter of your little frame. And if that boisterous Channel, and two hundred miles or so of land, come broad between us, I am afraid that cord of communion will be snapped; and then I've a nervous notion I should take to bleeding inwardly."
Live Action TV
In FlashForward (2009), three characters (Bryce, Keiko, and Olivia) see themselves in love with someone they haven't met yet in their flash forwards leading to two of them (Bryce and Keiko) on a quest to find each other and a third to doing everything she can to avoid her apparently-fated love interest. The recurring question of fate versus free will is never really resolved and it is heavily suggested that Olivia avoiding her match with Lloyd might actually doom the entire world.
The horror series The Vampire Diaries, which has Stefan and Elena's relationship. They are soul mates whose destinies are intertwined for eternity.
The 2012 series Touch, in which a young boy appears to be able to sense patterns in the larger world through numbers, specifically calls out to the Chinese legend, though implies connections beyond just love (a person saving someone's life, a person having an object another person desires but wouldn't have met otherwise, etc.)
In an episode of Star Trek: The Next Generation, an Enterprise crew member starts having dreams of a beautiful woman. When the Enterprise meets with a plague-infested ship, he finds the woman is there and has been having dreams of him, and created portraits of him. He decides to go with them to help them treat the plague.
The Irish band the Frames have a song called "Red Chord" on their album "Fitzcarraldo" that is based around this idea.
The cellist and singer Kanon Wakeshima, has the song named: L'espoir ~Mahô no Akai Ito~ (The Hope, Magical Red String) the song talks about a girl trying to get a guy to love her and talks about the Red String as a trap for her love, the song opens with this verse:
In the song "Adolescence" by the Vocaloids Kagamine Rin and Len, a mention is made about how it seems Rin and Len's hands are tied together with a thread.
"Just Be Friends" by Vocaloid Megurine Luka. Most of the pictures have her and another boy tied together with a red string. Which just adds to the whole sadness of the video and the song...
This video, using "Makka Na Ito" by Plastic Tree, which is about a couple joined by a red string who aren't together anymore, possibly implying that one of them is dead.
Thievery Corporation's video for "That Time We Lost Our Way" has Lou Lou Ooldouz Ghelichkhani singing while following one, with Rob & Eric playing bongos & a squeezebox nearby.
The picture book The Red Thread by the children's author and illustrator Grace Lin uses this trope to tell a fairy tale about adoption, of all things. In this case, it connects the adoptive parents and their child; magic glasses which enable the parents to see it allow them to find her. It's pretty sweet if you don't read this article first and end up thinking "Wait, isn't that the thing that they use to justify idiotic relationships in anime?"
In 7th Sea, when a Fate Witch sees red strands between people with her sorte magic, those represent conflict (Swords). Blue actually represents romance (Cups).
Changeling: The Lost uses red bands in a person's aura as a signifier of a pledge. When people who've made a pledge are near each other, the bands are connected with red threads.
In Werewolf The Apocalypse, the long-dead Apis shifters served as Gaia's matchmakers. One power of theirs let them see the red threads that bound people with a common destiny - though they couldn't tell whether it would be good or bad.
The recent Pokemon games have a red ball of string as an item (called "Destiny Knot" in the English version). Appropriately, if the monster holding this item becomes infatuated, the foe that it "loves" will be infatuated as well.
In the Anime based fighting game Naruto, Narutimate Hero Accel 2, the character of Chiyo has an Ougi where her attack puppets (Mother and Father, which are actually modified exhumed corpses) are joined by a literal red string, which is then used to viciously lacerate the opponent.
In The Legend of Zelda: Skyward Sword. In an interesting bit of Foe Yay during their second battle, Ghirahim appears again to tell Link that the reason they keep bumping into each other is because they are bound by a red thread of fate. He also gets behind Link, invading his personal space again, and whispers in his ear. He tells Link to come to him after licking his lips and says they're bound by that red thread of fate, that they're destined to fight. He also says that the thread of fate will be soaked crimson with Link's blood.
In Ilivais X, Iriana created a Blood Oath bond with Mille, making the string out of each other’s blood in a kind of haphazard transfusion. Since every cell of Iriana is designed to return to her, this essentially means Mille will always innately know where she is and be drawn to that location.
This lovely piece of art here. Perhaps not a traditional example, but it's there, and it's most likely romantic.
In contrast, a red string in India is usually a Rakhi and is a physical demonstration of a bond between a brother and sister. It can be tied to any boy a girl considers to be like her brother and is usually used effectively to kill unwanted romantic/love interests.
The Red String belief is so well known in Japan that this trope is invoked through linguistic gestures: holding up the thumb is used as shorthand for girlfriend and holding a pinky up would indicate a boyfriend.