The Portuguese parliament, which swung left at last month's general election, backed a law to allow same-sex couples to adopt, in a first reading of the bill on Friday.
The vote was passed with the backing of all the parties of the left along with some 20 deputies from the centre-right Social Democratic Party.
Luisa Corvo, head of the Portuguese gay rights group ILGA hailed the parliamentary vote as "a very important step towards allowing families to realise their adoption plans."
The law will "also better protect the many children who live within families that have not waited for the law to be adopted," she added.
In Portugal any individual already has the right to adopt but the law which allows same-sex marriage, passed in 2010, explicitly bans gay couples from adopting children.
"The fact that having homosexual parenting has no harmful effect on children has been scientifically established," the socialists said in their proposal to change the law which points out that same-sex couples are already allowed to adopt in 18 countries.
It was the fourth time since 2012 that the Portuguese parliament has been called on to rule on the subject, which the Socialist Party made an electoral promise.
The outgoing centre-right coalition lost its parliamentary majority at last months elections.
Portugal is currently in political limbo with the leftwing opposition waiting the green light to take power.
President Anibal Cavaco Silva is set to decide whether to charge Socialist leader Antonio Costa with forming a new government in the face of concerns over whether his fragile leftist alliance can last.
On Friday the parliament also repealed a series of amendments to the abortion law, including dropping charges introduced in July.