The 31-year-old, who won 52 caps for Germany and also played for West Ham and Everton, made the announcement in newspaper Die Zeit.
Thomas Hitzlsperger is the most prominent footballer to publicly reveal his homosexuality and said it was “a good time” to do so.
“I’m coming out about my homosexuality because I want to move the discussion about homosexuality among professional sportspeople forwards,” he added.
The midfielder, who retired from the sport in September after a series of injury problems, said he has only realised “in the past few years” that he would “prefer to live together with another man”, adding: “I’ve never been ashamed of the way I am.”
However, he said the issue is taboo inside the dressing room and it had not always been easy to live with some of the comments dished out on the subject.
“Just picture 20 men sat around a table together drinking – you’ve just got to let the majority be, just as long as the jokes are halfway funny and the talk about homosexuality doesn’t get too insulting,” he said.