Chancellor Angela Merkel’s ruling conservatives were on course to form a grand coalition government.
Germany’s left-of-centre Social Democrats voted in favour of joining a ruling alliance, in a nationwide ballot of the party’s grassroots membership.
Germany’s new coalition of conservative Christian Democrats and their traditional opponents is set to be sworn in early this week.
Sigmar Gabriel, the jubilant Social Democrat party leader who had staked his political future on a “yes” vote, described the outcome as a “celebration of internal party democracy”.
The Social Democrats will be the junior partners. But they have already won concessions including the introduction of an €8.50 per hour minimum wage.
In a surprise development, the defence minister’s post looks set to go for the first time to a woman – the conservative social affairs minister, Ursula von der Leyen, 55. She has also been tipped as a possible successor to Ms Merkel, should she step down in 2017.