De Rossi (left) and DeGeneres began dating in 2004
US talk show host Ellen DeGeneres has bought $100,000 of TV airtime to back gay marriage.
The 50-year-old will urge Californians to vote to save the right to gay marriage during the state ballot in November, campaign organisers said.
Same-sex unions were legalised in May, but Proposition 8 seeks to define marriage as between a man and a woman only under the state constitution.
DeGeneres married her partner, actress Portia de Rossi, in August.
Her TV advert will be screened in the run-up to the state ballot on 4 November.
DeGeneres will tell viewers that opponents of gay marriage are "twisting the truth", and will appeal to people to vote for "compassion and fairness".
Recent opinion polls have shown a swing in the number of Californians who oppose same-sex marriage.
Opponents of have stepped up their own TV advertising campaign, warning that children will be taught about gay marriage in primary schools.
Gay marriage supporters have called that argument misleading.
DeGeneres made broadcasting history in 1997 when her TV alter ego came out of the closet on her self-titled sitcom Ellen, becoming the first openly gay lead character on US prime-time network TV.