"Proud to be a happily married bisexual mother. Marriage is about love not gender," the True Blood actress, 31, wrote on Twitter, nine days into LGBT Pride Month.
She then linked to the Twitter accounts of Equality California, the NOH8 Campaign and the "It Gets Better" initiative, all of which fight for LGBT rights.
Paquin came out as bisexual in 2010, just months before marrying her costar Stephen Moyer. She did so in a PSA for the True Colors Fund's "Give a Damn" campaign, also dedicated to LGBT equality.
In 2012, the Oscar winner told Zooey magazine that her sexual orientation was a "minor biographical detail," but that she felt compelled to speak out for LGBT rights.
"If you're going to talk about some cause in a way that's meaningful, you should identify why it means something to you," she said. "For a bisexual, it's not about gender. That's not the deciding factor for who they're attracted to."