"Because no one has the right to deny another their life, even though they disagree with it, because everyone has the right to live the life they so desire if it doesn't harm another and because discrimination has no place in America, my vote will be for equality and against Proposition 8," Us magazine quoted the actor, as saying in a statement.
Pitt's donation is the largest thus far to the anti-Prop 8 campaign by an A-lsit celebrity, the L.A Times reported.
"It is our hope that others in the entertainment industry will step up and match Brad Pitt's heroic commitment to quality and defeating Prop. 8," Political strategist Chad Griffin said in a statement.
Pitt has said he won't wed Angelina Jolie until gay marriage is made legal.
"Angie and I will consider tying the knot when everyone else in the country who wants to be married is legally able," the actor told Esquire.