BRUSSELS — Martin Schulz, the president of the European Parliament since 2012, announced on Thursday his return to German politics, fueling speculation that he might challenge Chancellor Angela Merkel, who plans to seek a fourth term in elections next year.

The announcement by Mr. Schulz, 60, had been expected. Speculation about his return to German politics gained new momentum after Frank-Walter Steinmeier, Germany’s foreign minister, was tapped to become the country’s next president.

Mr. Schulz said he would seek a seat in the federal Parliament from North Rhine-Westphalia, the most populous state in Germany. From there, he might lead the Social Democratic Party’s challenge to Ms. Merkel’s party, the center-right Christian Democratic Union, in elections scheduled for September. The two parties have governed Germany in a “grand coalition” since 2013.

Another possible scenario is that Mr. Schulz would replace Mr. Steinmeier as foreign minister, a possibility if the Social Democrats were to select their chairman, Sigmar Gabriel, currently the vice chancellor, as the party’s candidate for chancellor, the head of government.