Mary Katherine Gallagher is a fictional character invented and portrayed by Saturday Night Live star Molly Shannon.

Mary Katherine is a sardonic caricature of an unpopular, teen Catholic School Girl. She is prone to comically severe bipolar mood swings, alternately spastic and ponderous. The character began on SNL in popular sketches that would generally unfold in the same manner: Mary Katherine would attend some sort of school-related function where she didn't fit in. She would attempt to participate, and seemingly do an unexpectedly good job, until she would lose her cool and do something disgusting. Ultimately she would spaz out and end up destroying something or falling on her behind. She would then jump up, extend her hands in the air, and proclaim herself a "Superstar!".

The character was popular enough to produce a feature length film, entitled Superstar. It featured Shannon as Gallagher and a cast of mostly adult actors portraying extreme versions of archetypical high school students. In the film, Mary Katherine lives with her disabled grandmother. She has the modest dreams common of most misfit youngsters: wishing to kiss a popular boy, fit in better, win a school talent contest. Naturally, mishaps occur.

Catch phrases attributed to the character:

• "Superstar!"
• "Sometimes, when I get really nervous, I put my hands under my armpits, and then I smell them like this..."