Simply put, two characters who aren't in a romantic relationship pretend to be.
Supertrope of Undercover as Lovers, Operation: Jealousy, The Beard, Citizenship Marriage, Shock Value Relationship, Boyfriend Bluff, Romantic FakeReal Turn. Please only add examples that don't fit on these subtropes.
- In Scum's Wish, Hanabi Yasuraoka and Mugi Awaya are going out together, but they both love someone else — their teachers.
- Ranma ½:
- One of these occurs after Ukyo mistakes Ranma's guilt induced kindness as affection, leading her to try getting closer to him and even start living with him. So in order to get Ukyo to leave, Ranma and Akane are forced to pretend to already be married, when they're actually just reluctantly engaged at that point. It doesn't fool Ukyo though who keeps making moves towards Ranma.
- This occurs during one of the most confusing love triangles ever to exist. Ukyo finds herself being pursued by Tsubasa. So in order to try and get rid of Tsubasa, seemingly a girl who loves dressing up, Ukyo pretends to be in a full blooming relationship with Ranma (they are supposedly engaged, but haven't become serious). However, given Ranma is a Sex Shifter and Ukyo dresses like a boy, she told Tsubasa that she was with Ranma's girl form. Upon finally being rejected by Ukyo enough times, Tsubasa gives up and starts falling for girl Ranma. The kicker is that, Tsubasa is actually a boy crossdressing and always knew that Ukyo was secretly a girl, rather than a girl attracted to girls as Ranma and Akane had initially thought. Meanwhile Ukyo always knew that (because they first met at a boys only school) Tsubasa was a boy. But they never asked.
- In Koi to Uso, there are severe social consequences for not going through with your Bureaucratically Arranged Marriage. Yajima and his arranged fiance feel no connection and haven't willingly spent time together in years (in large part because he still loves his ex-girlfriend), but both claim to be in love and ready to marry any day because otherwise their careers would be in jeopardy.
- In Mahou Sensou, the protagonist Takeshi Nanase established a fake relationship with his childhood friend Kurumi Isoshima in order to ward off numerous men who pursue her, especially after a traumatic incident with a stalker.
- In SPYXFAMILY Lloyd and Yoru start a fake relationship for their own reasons: Lloyd wants to get his daughter Anya into an elite school and will look more legitimate with a wife, and Yoru wants to escape the scrutiny cast on unwed women in the ultra-paranoid Cold War-esque climate of their country. The twist is Lloyd - aka the elite spy <Twilight> - wants to get his "daughter" into the school to get close to his mark, while Yoru is trying to avoid unwanted suspicions because she's actually the deadly assassin <Thorn Princess>.
- In Red Robin Tim wakes up after being treated for injuries in the Batcave to have Damian shove a newspaper in his face declaring to a shocked Tim that Tim is engaged to Tam Fox. Tam had panicked while being questioned by Vicky Vale and blurted out that they were engaged to redirect her line of questioning. They keep the charade going for a week or so because they're trying to distract Vicky from her attempt to uncover the Batfamily's secret identities.
- The whole plot of Roadqueen: Eternal Roadtrip To Love revolves around Leo and Vega "fake-dating" for a week in order for Leo to prove she can be a great girlfriend and "decent lesbian" so Vega will return her beloved motorcycle.
- In Victor/Victoria, when Victoria starts masquerading as the female impersonator Victor, she and Toddy pretend to be lovers.
- In the Warrior Cats book Bluestar's Prophecy, Bluefur became pregnant as the result of a Forbidden Romance with a cat from another Clan. Her Clanmate Thrushpelt - who she was friends with and who had unrequited feelings for her - offered to pretend that he was the father in order to help her out. Bluefur decided to not outright tell everyone that he was, Thrushpelt still loved the kits as if they were his own and behaved as if they were, and both of them were fine with everyone assuming that they were mates.
- Get Smart: Two KAOS agents once pretended to be in love, sharing a public kiss to escape detection by CONTROL. Max and 99 likewise often masqueraded as a married couple on missions before they got hitched for real.
- Booth and Brennan did this several times on Bones before getting together for real. They went undercover as "Buck and Wanda" a couple of times, and once they did it at Brennan's school reunion.
- The premise of the britcom Spaced is that the main characters believe they have a Single-Issue Landlord who will only rent the flat to a couple. It eventually turns out this was all based on a misprinted advertisement.
- Frasier pretends to be Roz's date at a family reunion when she's recently been dumped and can't face her competitive, passive-aggressive sisters.
- Martin pretends to date the mother of Frasier's Sitcom Arch-Nemesis as a prank on the pair of them.
- In the Doctor Who episode "The Time of the Doctor", Clara accidentally told her relatives that she had a boyfriend, leading her to call the Doctor in a blind panic.
Clara: Emergency: you're my boyfriend.The Doctor: Ding dong, okay, brilliant! I may be a bit rusty in some areas but I will glance at a manual!Clara: No no, you're not actually my boyfriend.The Doctor: (disappointed) Oh, that was quick. It's a rollercoaster, this phone call.Clara: But I need a boyfriend really quickly!The Doctor: Well, I hope you're nicer to the next one.
- The Wire: Cedric Daniels actually was in a relationship with his wife Marla (obviously), but they have secretly broken up just when her career in politics is about to take off. Daniels is actually involved with Pearlman at that point, but for public appearance's sake, he still shows up to a dinner his wife is hosting for several important guests before going upstairs to wait until they're gone.
- In a season one episode of Hannah Montana, Miley tells Jake that she's dating a graduate student named Will, who has agreed to pose as her boyfriend. Unfortunatly for her, Will is 11 years old, graduated from elementary school, and only agreed to the pretend relationship when dared to by his friends.
- Fry on Futurama gets himself into a fake relationship triangle. First he pretends to be Amy's boyfriend so that her parents would stop trying to set her up with men. Later he pretends to be Leela's boyfriend so that Zapp Brannigan will stop hitting on her. This becomes awkward when all of them are having dinner at the captain's table in front of both Amy's parents and Zapp.
- South Park: In the episode "About Last Night", it is revealed that Barack Obama and Michelle Obama aren't actually married, but it is all a ruse for getting Obama into office so he and McCain can steal an expensive diamond. At the end of the episode, Barack decides to be president after all and asks Michelle to stay with him.
- In The Proud Family, Penny and Sticky get into one of these because the former (begrudgingly) wanted to assist the latter in gaining back Dijonay's affections for him.
- In Goof Troop, Max convinced his attractive older cousin, Debbie, to pretend to be his girlfriend in an attempt to impress the cooler kids. It worked, though with the added complication of one of them trying to "steal" her from Max.
- In Disenchantment, Elfo makes up a Girlfriend in a Faraway Land to try to distract from his obvious feelings for Bean. Unfortunately, Bean wants to "reunite" them, and her knights bring back Tess, a giantess who happens to fit the description Elfo gave. Elfo begs Tess to maintain the lie through the upcoming royal ball.
- In BoJack Horseman, Princess Caroline makes Todd pretend to be dating Courtney Pourtenoy in order to give her a down-to-earth image and promote her new movie. It backfires as Todd accidentally becomes a famous model.