The Shawshank Redemption star narrates the commercial for the Human Rights Campaign, whose bosses argue recent ballot box victories have provided a nationwide "mandate for full equality".
Voters in Maryland, Maine and Washington passed measures earlier this month (Nov12) to confirm the legality of same-sex marriage, but in Minnesota voters rejected a ban on same-sex unions.
In the new advert, Freeman says, "America stands at the dawn of a new day. Freedom, justice and human dignity have always guided our journey toward a more perfect union. Now across our country, we are standing together for the right of gay and lesbian Americans to marry the person they love."
The commercial is due to air in the U.S. on Sunday (02Dec12).
Freeman is no stranger to narrating high-profile ads - he voiced a film for President Barack Obama's re-election campaign in October (12) detailing the incumbent's first-term accomplishments.