The Fight Club star has been a long-time activist for same-sex unions, famously saying he would not marry fiancee Angelina Jolie until everybody in the U.S. had the right to wed.
Pitt has encouraged fans to donate cash which he will match dollar for dollar in order to raise awareness and support for marriage equality issues, which is on the ballot in four states during next week's (beg06Nov12) U.S. election.
In a statement, the actor says, "It's unbelievable to me that people's lives and relationships are literally being voted on in a matter of days. In Maine, Maryland, Minnesota and Washington, voters will go to the polls to decide if gay and lesbian couples - our friends and neighbors (sic) - are worthy of the same protections as everyone else.
"But that's the system we have and I'm not going to back down from the fight for loving and committed couples to have the ability to marry. Especially when groups like the Human Rights Campaign are fighting these battles day-in and day-out.
"Every person's right to life, liberty, and the pursuit of happiness is enshrined in our country's Declaration of Independence, but powerful, well-funded groups are flooding the airwaves with lies trying to take away those rights from certain people... and we can't stand for it."
HRC President Chad Griffin says, "With his commitment, Brad joins HRC in a tremendous coalition of religious leaders, business leaders, labour groups, civil rights organisations and everyday, fair-minded Americans supporting marriage for gay and lesbian couples."