The Social Democrat, 61, is one of Germany's most popular politicians.
The post is largely ceremonial, but the president represents Germany abroad and is seen as carrying moral weight.
During the US election campaign, the usually circumspect ex-lawyer described Donald Trump as a "hate preacher" and predicted more challenging relations with Washington.
He has also criticised those who "make politics with fear", and spoken out against right-wing populism.
'Straight-talking'
