Lady Looks Like a Dude

An unfeminine female character looks like, or is mistaken for, a male. Sometimes this is to show that she's as tough as a guy, sometimes to show that she's a Butch Lesbian, but frequently just to show that she's unattractive. Often used for comedy, in which case she may be an Abhorrent Admirer.

Supertrope of Bifauxnen, where a woman looks like an attractive young man, usually with fangirls (or fanboys) who take obvious notice. Not to be confused with Sweet Polly Oliver, who is a women deliberately disguised as a man; Samus Is a Girl, where the story is deliberately misleading about the character's gender; or Viewer Gender Confusion, where there's only out-of-universe confusion. Compare Dude Looks Like a Lady and Attractive Bent Gender, for men who look like women, and The Ladette, for women who act like men. Sometimes involves a Gender Reveal.

Anime and Manga

 * Project A-ko has a female One-Gender Race of Human Aliens. Many look like guys.
 * Cowboy Bebop: Ed is a girl, which the rest of the cast doesn't realize until the very end of the episode in which she's introduced. Given that she's young enough to be prepubescent and her personality is halfway between Feral Child and Cloudcuckoolander anyway, it doesn't actually affect her characterization much.
 * Gundam Seed: Cagalli is introduced this way, though she wasn't specifically disguising herself as male, people assume she is until it's revealed otherwise.
 * Hellsing: Sir Integra, who dresses in a masculine fashion and is referred to as "sir" despite being female. It's explained that this is deliberate on her part, to make people more accepting of her taking the traditionally male role of the head of the Hellsing Organization.
 * Zorin Blitz is an even more convincing example.
 * Magical Girl Lyrical Nanoha Striker S: The supplementary manga revealed that Otto of the Numbers Cyborgs had such a boyish appearance that most of her own Cyborg family were unsure if she had a male or female body.
 * Very early on in Ouran High School Host Club, where is assumed to be male because of her very unflattering appearance. This continues through the rest of the series, though a proper clothing upgrade makes her into more of a Bifauxnen.
 * Ai Ore Love Me: Sakurazaki Mizuki. The other girls of her band also fall into this trope. One person even stated nobody in that band looks like a girl except one member. The Irony is that the one who looks like a girl isn't one.
 * Sailor Moon: Haruka was first thought to be male by the protagonists until she appeared in a dress in the end of her debut episode.
 * Heartcatch Pretty Cure: Both Tsubomi and Cobraja thought that Itsuki was a male until different circumstances revealed the truth (a servant of Itsuki's called her "Oujo-sama" in front of Tsubomi and Erika/Cure Marine told Cobraja flat out that Itsuki was a girl). Cobraja was shocked while Tsubomi ended up falling slightly ill from the revelation.
 * VT the middle-aged space trucker in Cowboy Bebop.
 * Liechtenstein of Axis Powers Hetalia cuts her hair short to mimic her adopted brother Switzerland. When the two go walking while wearing the same military uniform, an old lady compliments Switzerland on having an adorable younger brother. Cue Liechtenstein groping her chest in shock. This isn't as much of a problem when she wears dresses and a ribbon.

Film

 * In Sky High, Royal Pain is thought to be a guy because of the voice modulator. It is later revealed that she is a she when she finally puts her evil plan into action.
 * Madame Gasket in Robots is mistaken for a man, even down to the voice. The Reveal that she is in fact a woman has some pretty hilarious reactions (including the glass that helps protect the eyes shattering).

Live Action Television

 * Monk: Mr. Monk gets stuck in traffic, where Monk runs into a truck driver whom he initially assumes to be a male.
 * In Married... with Children, Marcy was mistaken for a male several times due to her flat chest and short hair.

Western Animation
"Patty: When you do good, I use the green pen. When you do bad, I use the red pen. Any questions? Otto: Yeah, one. Have you always been a chick? I mean I don't want to offend you, but you were born a man, weren't you? You can tell me; I'm open-minded. Patty: (Dropping the green pen) We won't be needing this."
 * Family Guy:
 * The Griffins go into witness protection in the Deep South. Chris befriends a kid named Sam who looks and sounds like a guy -- until he kisses Chris and it's revealed shortly after that he is a she.
 * Another episode, "Mind Over Murder", the 'man' Peter punches turns out to be a woman, leading to her delivering her child ahead of schedule.
 * A recurring joke is that Meg is mistaken for a boy (despite heving a clearly female chest).
 * The Simpsons, episode "The Otto Show": Otto has to take a driving test, and Patty is his tester.

"Priest: He was a caring man, he was a kind man. He gave to his community and asked little in return. He- [A man whispers something in his ear.] That's a woman? Oh, dear God!"
 * The episode "Selma's Choice" has a moment where the priest is eulogizing the deceased Aunt Gladys at her funeral:


 * Todd World, a series targeted at preschool audiences. Todd's dog, Benny, meets a dog named Sam and they play together in the mud, so he never really gets a good look at her. When they meet later, Benny is shocked she's a girl-- "Sam" is short for Samantha. He thinks that he can't play with a girl, but learns differently. "Benny has a new friend now. Girls make great friends and so do boys!"
 * Avatar: The Last Airbender: Smellerbee is frequently mistaken for a boy, a misconception which she is somewhat sensitive about and quick to correct.
 * In the episode of Hey Arnold! where Helga gives herself a makeover a man mistakes her for a boy.

Video Games:

 * (possibly:) The Legend of Zelda: Ocarina of Time: Shiek is a debatable example. It's unclear if is magically transformed, deliberately disguised, or simply dressed in a masculine fashion when appearing as Shiek.
 * Touhou: Wriggle Nightbug is female, like 99.99% of the cast, despite her androgynous appearance (to the point of looking like a cute boy sometimes) and all the fanwork to the contrary.
 * Pokémon: The Machop and Timburr lines have a 25% chance of being female, despite their very manly appearances- the latter line's final evolution, Conkeldurr even sports a visible goatee!
 * from Solatorobo, at least according to the characters in-game. This even allows Red to help her sneak into an all-boys magic school, insisting to the Ritual Master that she "couldn't be any more of a boy" - which, of course, annoys her immensely, since by that point she was quite fond of him.
 * Viewers tend to get confused when first introduced to her: since she wears a Skirt Over Slacks and has an androgynous face, most players assumed she was female, yet other characters insisted on referring to her as a boy and she uses masculine pronouns in the Japanese version, so players assumed they must've been mistaken. Then it's revealed that she is a girl after all; that Skirt Over Slacks look is apparently boys' clothing in the Shepherd Republic, which is why the other characters thought she was a boy. She never bothered to correct anyone because they just never asked which she was.
 * Adel. The only clue that she is a she is that she is called a sorceress.

Web Comics:

 * In Hemispheres, Mineshaft, the dwarf, looks like a generic fantasy dwarf male... but, of course, it's eventually revealed that dwarves in this setting all look and sound male to humans, beards and all.

Real Life:

 * Mary Frith, a.k.a. Mad Moll Cutpurse was a well-known pickpocket and pimp in the English Renaissance, who dressed as a man, dueled other men, smoked a pipe, and swore. She was married to a man eventually, but it was largely believed to be a domestic farce. Thomas Middleton's play The Roaring Girl was based on her.
 * Kim and Kelley Deal. Wearing lumberjack shirts and smoking like chimneys hasn't helped
 * Anne Bonney and Mary Read, female pirates in the early 1700's. Sources differ on if or how long they disguised theirselves as men, though.
 * Patti Smith
 * Jeanne d'Arc (known largely as Joan of Arc in the U.S.) cut her hair and dressed as a man during the Hundred Years War. Actually, when she was on trial by the English for heresy, this was one of the only things they could convict her of, and thus was one of the sole reasons she was burned at the stake.
 * In Britain, during times when hairstyles either favored shorter hair for women or longer hair for men, it could be hard to tell the difference between a man and a woman who just wore trousers (or for pre-adolescents, a girl could easily be mistaken for a boy who hadn't hit puberty).
 * Female spotted hyenas look almost exactly like the males, right down to having an enlarged clitoris that's essentially a pseudopenis, complete with a fake scrotum. And they give birth through it.