The law was passed by the German parliament (Bundestag) at the end of June. The legislation states that the reform will come into force “on the first day of the third month following the declaration”, meaning likely in October, according to weekly newspaper Die Zeit.
The newspaper further explained that state registry offices need time to adapt to the change.
Steinmeier's signature comes amid gay pride celebrations across Germany, with Berlin's grand Christopher Street Day parade set to kick off on Saturday.
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The Bundestag's vote right before the parliament went on its summer break came as a surprise to many, and followed Chancellor Angela Merkel advising her conservative party colleagues to vote based on their conscience rather than the party line.
Merkel herself said she voted against the reform.
Leading members of the conservative CSU/CDU parties (also known as the Union) still remain displeased with the vote and have called for the law to be challenged before Germany's Constitutional Court.
Bundestag Vice President Johannes Singhammer of the CSU urged his state of Bavaria earlier this month to file a suit against the law. Singhammer and others argue that the reform should require a constitutional amendment, and therefore would need a two-thirds majority to pass - something unlikely in a parliament where nearly half of the members belong to the Union.
Article Six of the Constitution regarding marriage does not explicitly mention gender in its definition, stating simply that “marriage and the family shall enjoy the special protection of the state”. But the Constitutional Court has repeatedly interpreted the Constitution to mean that marriage only applies to heterosexual couples.
Bavaria itself - whose Minister President is Horst Seehofer, the head of the CSU - has said that it is considering filing a legal challenge and is consulting with legal experts.
Volker Kauder, leader of Merkel's CDU in the Bundestag, told Tagesspiegel earlier this month that his party is going to depend on Bavaria's assessment of whether to file a complaint.
Cardinal and president of the German Bishops' Conference, Reinhard Marx, last week also said that he would “very much welcome” a legal challenge by his state of Bavaria.
“A ruling would be very good for public peace in Germany,” Marx told the Augsburger Allgemeine.