Sometimes people just don't get the idea of jokes or lies and are seemingly tuned to believe anything people tell them. They might be The Ditz, very pure or what have you, but the net result is that when someone tells them ridiculous stories that no sane person would find even slightly credible they'll believe it wholeheartedly. Sometimes, however, it actually works in reverse: A character will tell them something that is completely true but rather strange and the character will refuse to believe it.
- Hentai Ouji to Warawanai Neko: Tsukushi Tsutsukakushi is the strict president of the track and field club, but she is so gullible that she believes Youto's ridiculous excuse that he's innocent of the perverted acts she's determined to punish him for because they were actually committed by his (made-up) identical twin brother. As a result she continues to trust him like she always has while attributing anything naughty he does to his twin brother, and when she catches him in an Accidental Pervert situation she takes him for his brother and refuses to believe him when he tries to explain that the "twin brother" thing was a lie in the first place.
- Ayaka in Mahou Sensei Negima! tends to believe a lot of what she hears given how naïve she is. At one point Chisame needs to distract her and the only thing she can come up with is that Chao is Negi’s fiancée and they’ll have to get married if he loses to her, lampshading how absurd her lie is. Ayaka buys it. She also believes her when she says her Magitek wand that brings them inside the world of the computer is just how video games work now.
- Wakamatsu in Monthly Girls' Nozaki-kun is adorably naïve and willing to believe what people tell him. Even the nicer characters like Sakura and Hori will do it, such as claiming there’s no seniority in culture clubs, finding out they can’t apply screentones while Wakamatsu can and then instantly and arrogantly shoving the work off on him, completely reversing what they had said before.
- Youko in My Monster Secret is as terrible at telling lies as Asahi, but also falls for them even more easily than he does. And when Karen is introduced, she turns out to be such a Nice Girl that the words "doubt" and "suspicion" seem thoroughly absent from her dictionary.
- This is pretty much the hat for the whole Tontatta Tribe in One Piece. Their tendency to believe everything people say has been exploited by heroes and villains alike. "Tonta" even means "foolish" in Spanish.
- Ranma ½: Ryoga's naivety and trusting nature makes him everyone's victim, especially Ranma, who repeatedly uses his girl form to either spy on him, or dupe him. You'd think Ryoga would eventually learn to recognize her face, or her voice - but he never does. Even when Ranma's using little more than a pair of glasses as a disguise. The most egregious instance being, when Ranma was even able to convince Ryoga that he was his sister. Ryoga figured it was plausible, given he hadn't seen his family in years, due to his poor sense of direction.
- Sailor Moon: In the first season finale, the DD girls create an illusion of a captured Tuxedo Mask, causing Usagi to run toward it. After the trap is exposed, they create the same illusion again... and Usagi does the same thing.
Sailor Mars: Yeah, right. Do you really think she's (Sees Sailor Moon running toward the illusion) stupid enough to fall for it AGAIN?''Sailor Moon: But what if it really is him this time?
- Servant × Service by the same author as Working!! has its own Popura expy with Lucy, who is incredibly willing to give out personal information for flimsy reasons or believe Chihaya’s terrible lies.
- Shimoneta: Oboro takes the concept of Just Following Orders to its ludicrous extreme. As head of the school's Decency Prefects, his job is to confiscate any material that's considered lewd. So when he tries to take Fuwa's yaoi doujin, Fuwa contends that it's research material. Oboro immediately believes her and authorizes the book. However, Tanukichi tells him it's smut, so Oboro promptly takes the book back and bans it. The gag continues back and forth like this for several moments.
- Popura from WORKING!! will believe any ridiculous lie Satou or Souma tell her, such as that eating dried geckos will help her grow taller. She also somehow fails to realize that Souta and Kozue knowing each other and having the same last name while living in the same neighborhood is probably a hint that they’re siblings.
- Kirie from World Trigger believes anything other people tell her, no matter how ridiculous. This includes believing Jin when he said Osamu, Chika and Yuma were his siblings when they don't look alike at all.
- Kido from Zatch Bell! always believes whatever bizarre things Dr. Riddles tells him and then Riddles responds with "It's a lie!"(or "Kidding!" in the dub) much to Kido's dismay.
- Sakura of Cardcaptor Sakura always believes any tall tale her classmate Takashi tells her without fail. Most new transfer students also tend to fall into this, which is eventually lampshaded by Takashi's girlfriend in Cardcaptor Sakura: Clear Card.
Chiharu: [while angrily throttling Takashi] YOU FOUND YOURSELF ANOTHER VICTIM! ARE YOU HAPPY NOW?!
- Played for Laughs in The Gamers: Dorkness Rising: Sir Osric, The Paladin (and a GM PC), tries to keep the party on a morally upright path, but whenever they need to do something illegal, they simply Bluff him into running out of the room to fight some non-existent evil. Because his Sense Motive skill is so low, apparently, it works like a charm every time.
- Lingk in the play and film version of Glengarry Glen Ross is not only an Extreme Doormat and Henpecked Husband, he also swallows every lie the Shady Real Estate Agent Roma feeds him without getting suspicious. Even when one of Roma's statements is exposed as untrue by a third person, Lingk's reaction is to apologize for letting Roma down.
- Justified in The Invention of Lying which is set in a world where no one has any concept of lying whatsoever, which means that people believe the protagonist (the only person able to lie) no matter how ridiculous his lies get. Lines like "I'm not here" and "we have to have sex right now or else the world will end" are taken at face value.
- The Muppets in Muppets Most Wanted are extremely gullible in this scene where Dominic Badguy introduces himself by flashing his business card. Questioned about his strange surname he claims it's French and means "good man", which the Muppets swallow in a heart beat.
- Woody in Nebraska - it's extremely depressing to see an elderly man get his hopes up over receiving a "Big Winner!" letter for a million-dollar sweepstakes.
- One of Isaac Asimov's Azazel short stories is about a police detective who believes the most ridiculous stories told by the suspects (like a shop robber saying that the owner gave him a gun and started putting money in his pockets). The titular demon makes him a Living Lie Detector to compensate... causes some problems with his girlfriend, but that's another matter.
- Everyone in Graceling becomes this when Leck is involved. Lying is his Grace, so even if the lies are several times removed from the source, people believe them despite how very suspicious they are. Po is the only one who can see through it.
- Angels in Unsong start out unable to even comprehend the idea of lying. Of course extended contact with humanity tends to eventually cause them to figure it out which also leads to them having breakdowns.
- Mr. Gullible the substitute teacher from The Amanda Show, as his name implies, is really gullible. When someone would tell him an obvious lie, he would ask them "Really?" and then he will believe the lie as long as everyone who's in on the lie agrees it's true.
- Barney Stinson from How I Met Your Mother is as an adult still believing all the ridiculous lies his mother has told him as a child. This is also quite ironic, considering that Barney is a notorious liar himself.
- Kaamelott. Arthur is repeatedly able to convince the chieftains of enemy tribes to go away thanks to this trope.
Arthur: I'll give you the entire Burgund territory.Viking: You wha...? And... he agrees with this?Burgund: HE AGREES WITH THIS!
- In one case, he tells a Viking allied with the Burgund king that the Burgund was giving up all his lands (the Burgund doesn't speak their language and tends to repeat what he just heard because it sounds funny to him) with this exchange:
- In another instance, the Burgund king is attacking Kaamelott when Attila the Hun shows up. Arthur then tells Attila the the Burgund is in fact here as his ally, despite the Burgund screaming "Attila! Attila!" in a panic and Arthur yelling at him to shut up before he screws everything up. Attila falls for it.
- And in another where the Mongol horde showed up (all two of them), Arthur convinced Attila that the castle didn't contain anything of value at all, to the point where Attila's demands kept going lower (at one point demanding the table linen, but Bohort refused). In the end he demands the queen, gets Arthur's Ax-Crazy bodyguard Grudu in obvious drag, and accepts her as the real deal.
- From Better Call Saul, Jimmy and Chuck's father was the epitome of Stupid Good, running his own business into the ground because he believed every sob story he was told no matter how blatantly false it was and handed out money and sometimes even free merchandise from his store to anyone and everyone who came to him with one.
- In Dungeons & Dragons and Pathfinder, characters with low Wisdom and Sense Motive/Insight modifiers tend to have trouble discerning truth from lies, and therefore often fall under this trope.
- The GURPS disadvantage Gullibility gives this trait to a character. They'll believe anything they're told, no matter how ridiculous it is.
- Ogryns in Warhammer 40,000 are Heavyworlder Psychopathic Manchildren that possess childlike faith in the God-Emperor. Unfortunately, as Belief Makes You Stupid, they are very gullible, and many rebellions are caused by the instigator telling the Ogryns the Emperor said their caretakers are evil.
- Valvatorez in Disgaea 4: A Promise Unforgotten will believe lies told about his own motives and actions. He also thinks that Axel is a noble compatriot who died in the line of duty when he was entirely and openly hostile to them throughout every appearance. Why? Because Fenrich said so.
Fenrich: My lord’s undoubting heart is another of his formidable abilities.
- Gatrie, the Nice Guy Armor Knight of the Greil Mercenaries from the Fire Emblem Tellius subseries is extremely easy to take advantage of, as revealed in his support conversation with his Heterosexual Life Partner and Vitriolic Best Bud Shinon. There, a woman he tries to woo manages to get all of his money to "pay for an operation" because she is "terminally ill" with "Brain Stones." She runs off, never to be seen again. In a twist, Gatrie is actually aware of his gullibility, but thinks that it makes him charming.
- This was originally Knuckles the Echidna's defining characteristic. Having grown up in total isolation, it was easy for Dr. Robotnik to convince him that Sonic and Tails were his enemies. As of late however, it's been Flanderized into general stupidity.
- Sunohara in CLANNAD consistently falls for Tomoya’s bullshit, even being convinced once that he was dreaming simply because he didn’t believe Tomoya would remember the capital of Australia. Or being woken up after class and being told that they were living in a post-apocalyptic wasteland and that Tomoya was a robot duplicate of the original Tomoya.
- Chris in Maji De Watashi Ni Koishinasai has this weird thing where she acts super suspicious of Yamato, but then believes whatever absurd ideas he tells her, even correcting himself to continue the lie after he gets caught. Interestingly, the one time she doesn’t believe him is when he tells her about the Japanese folk remedy involving sticking a spring onion up someone’s ass as a cure for colds and other ailments.
- Dangan Ronpa
- Played for Drama in Super Danganronpa 2. Near the middle of the third chapter, Ibuki Mioda catches a disease which Monokuma released on the island with random, personality-affecting symptoms. Among the three who contracted the disease, she was unlucky enough to get 'gullibility' as her symptom, which made it easy for Tsumiki -who was the nurse that was supposed to be caring for the sick- to strangle her to death; Mioda quite literally went along with her own murder without suspecting a thing.
- The side-story Danganronpa IF reveals Mukuro Ikusaba to be this regarding anything her sister (the Big Bad of the franchise) says when she needed to get a blood transfusion for Naegi but didn't know his blood type. Despite said sister being the reason he was bleeding out in the first place, Mukuro followed her instructions without suspecting a thing (repeated several times during that passage for emphasis on Mukuro's gullibility). She had to be told right to her face that she was being tricked, after nearly giving Naegi the wrong transfusion.
- As a result of having raised in the forest, Gonta Gokuhara from New Dangan Ronpa V3 is this as in the start of Chapter Two, he honestly believed Kokichi Oma when he stated that he was a honest and trustworthy man.
- While not to extent of Gonta, Tenko Chabashira from the same game is this as she will believe anything that was said by the person who taught her Neo-Aikido, such as that her Neo-Aikido powers can get weakened by getting too excited for holidays, eating more than three sweets per day, or touching men. Note that the last thing is what ended up contributing to her disdain for men despite her mentor also being male, and she was still gullible enough to not notice that contradiction.
- Yumina in Yumina The Ethereal is basically brainless, so she’ll easily buy into whatever lies Ayumu tells her. She’s also easily flattered into doing things she wouldn’t normally want to.
- Exterminatus Now. The Cerberus demon falls for the "pretend to throw the ball" trick. It is a dog, after all.
- Invoked in Acquisitions Incorporated season 6, when Binwin tries to figure out whether Danielle is pulling a fast one on him and rolls for Sense Motive, and Aoefel bursts out into laughter because Binwin's Wisdom modifier is -1 and he would probably believe whatever Danielle said—even if she were just making handfarts.
- Game Grumps. Danny is very susceptible to this. Sometimes he will catch on to when he's being fooled, but it doesn't take much to convince him otherwise.
- In early seasons of The Simpsons this was one of Superintendent Chalmers' characteristics in relation to the lies Principal Skinner told him. The greatest example of this is him believing Skinner's story that the northern lights were occurring in the kitchen, when it was really a really bad fire.
- Starfire from Teen Titans lacks experience with Earth culture and tends to accept explanations without a second thought, since the culture difference is so large that it's hard to tell crazy from crazy.
- Steven Universe: The Ruby Squad from "Hit the Diamond" accept absolutely every statement at face value, even if it's made by someone who couldn't be acting more unnatural if she tried, or is blatantly impossible, like the lie that their target Jasper is on Neptune.
Ruby Fusion: Why didn't you say so!?
- The Loud House: Leni Loud, being The Ditz and all.
- Dug in the Pixar short Dug's Special Mission. He happily accepts Alpha's painfully flimsy excuses about why the "special missions" that Alpha's sending him on are necessary (they're really Snipe Hunts meant to get him out of the way).
- Kaeloo: Stumpy and, on occasions, Kaeloo herself.
- One of the symptoms of autism is a lack of "Theory of Mind", or the inability to understand another person's point of view or mindset. This may lead to the person with autism believing nearly everything he or she is told, even if the information is obviously false to everyone else.