The True Blood star made the announcement in a public service video for the Give a Damn campaign, reports Radar Online.
The campaign video features celebrities promoting equality.
"I'm Anna Paquin. I'm bisexual and I give a damn," the 27-year-old says on camera.
The video was recorded on January 30 in Los Angeles.
Paquin's video spot is on behalf of the True Colours Fund, an equal rights group supporting gay, straight, bisexual or transgendered people.
Elton John, Kim Kardashian, Sharon and Kelly Osbourne, and Whoopi Goldberg are some of the celebrities involved in the effort.
Paquin is engaged to True Blood co-star Stephen Moyer.