The nation's largest gay rights group said Wednesday that Pitt agreed to match contributions from members up to $100,000.
It was a rare political donation for Pitt. In an e-mail to members of the Human Rights Campaign, Pitt wrote that it's "unbelievable" that people's relationships will be put to a vote on Election Day.
Same-sex marriage will be on the ballot in Maryland, Maine, Minnesota and Washington state.
The Washington, D.C.-based Human Rights Campaign says it has spent $8 million to push for marriage equality for gays and lesbians over the past two years, including $5 million in the four ballot measures this year.