"I'm Anna Paquin. I'm bisexual and I give a damn," she said to camera during a private taping in Los Angeles.
The revelation sparked such a flood of visitors to the giveadamn.org website that it crashed, leaving a message that read: "Ok - we didn't expect THIS kind of traffic!"Paquin's contribution is included in the PSA alongside appearances from Sir Elton John, Clay Aiken, Whoopi Goldberg, Wanda Sykes and Sharon Osbourne.
The campaign promotes the rights of gay, bisexual and transgendered people.
Paquin explains in the video: "One hate crime is committed approximately every hour of every day in this country."
The 27-year-old actress announced her engagement to Moyer in August last year. They met on the set of True Blood, HBO's hit vampire drama, in which she plays waitress Sookie Stackhouse.
She found fame as a child star in 1993 film The Piano, becoming the second youngest Oscar-winner in history when she was named best supporting actress aged 11.
Paquin went on to appear in Fly Away Home and Amistad before getting her first blockbuster role as Rogue in the X-Men movies.
She relishes her role in True Blood, describing it as "a really full-on sexy show... lots of weird, kinky vampire sex".
In a recent interview, Paquin said she and British-born Moyer have no plans. "I'm not really that girl, we're not really that couple," she said. "I wasn't the girl who spent hours dreaming of my wedding."
