Steinmeier, a former foreign minister, was elected earlier Sunday as Germany's new president, a post more ceremonial than executive.
Mr Steinmeier succeeds Joachim Gauck, a 77-year-old former pastor and East German pro-democracy activist who did not seek a second five-year term because of his age.
Steinmeier, Germany's former foreign minister, was elected Sunday in Berlin with 931 of the 1,260 votes.
In his acceptance speech, Steinmeier said that Germany should be an "anchor of hope" at a time when democratic institutions were under threat across the world.
On Saturday evening, Angela Merkel and Horst Lorenz Seehofer, representing the center-left and center-right political union, called for support for Steinmeier, saying he would be the right person to represent Germany.
His election is likely to be one of the last moments of coalition unity ahead of a parliamentary election in September in which Ms Merkel is seeking a fourth term.
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A few years ago, Steinmeier took a several-months absence from politics, to donate one of his kidneys to his wife Elke Buedenbende.
Many people see Mr Steinmeier as the most popular poltician in Germany.
During the U.S. election campaign, the usually circumspect former lawyer described Donald Trump as a "hate preacher" and predicted more challenging relations with Washington.
A policy wonk by nature, Mr Steinmeier served as adviser and then chief of staff to Dr Merkel's predecessor, the SPD's Gerhard Schroeder.
Steinmeier was the Social Democractic Party candidate who challenged Angela Merkel in the Federal Election in 2009, and the chancellor gave her backing to the elected man. He also stated that the relationship between Germany and the United States could become "challenging", according to the BBC.