Outgoing Portuguese President Anibal Cavaco Silva on Monday in Parliament vetoed the introduction of adoption rights for homosexual parents, saying children's welfare had to take absolute priority.
It had not been proven that adoption by homosexual couples was in the children's best interest, the president said as he sent back the law, which had been passed with the votes of the left and several legislators of Cavaco's liberal-conservative PSD party in November.
Gays and lesbians in the Catholic country had celebrated the reforms as a great success.
The governing Socialists and Communists said they would put the vetoed law up for another vote.
Cavaco is due to leave office on March 9. He will be succeeded as president by Marcelo Rebelo de Sousa, who won an election on Sunday.
Homosexuality was a punishable offence in Portugal until 1982. Same-sex marriage was made legal in 2010 despite heavy protests by the Catholic Church.