Spanish Prime Minister-elect Jose Luis Rodriguez Zapatero says he is in favour of allowing same-sex marriages.
Zapatero had made a campaign pledge on the subject
His government will draft a law to put gay unions and marriage on an equal footing, he told Spain's Telecinco television channel on Thursday.
He said same-sex marriage was a characteristic of a "modern and tolerant society".
The conservative outgoing government of the Popular Party had rejected several calls to allow same-sex marriages.
Mr Zapatero swept to a surprise victory in general elections held on Sunday.
Legalising gay unions was one of his campaign pledges.
In the interview he did not set a time-frame for the change.
Spain is an overwhelmingly Roman Catholic country and the Vatican condemns same-sex marriages.