Schulz, 60, "will make a switch to Berlin", reported Sueddeutsche Zeitung daily, which said the German veteran politician will not seek a third term as European Parliament chief.
Spiegel Online also said he "will make the announcement on Thursday morning in Brussels".
Public broadcaster ARD later said it had obtained a confirmation from Schulz about the reports of his plans to leave Brussels.
However, it remains unclear if Schulz will be the lead candidate for his Social Democratic Party (SPD), currently junior coalition partner in Merkel's grand right-left government.
Schulz's name has also been circulated as a potential replacement for German Foreign Minister Frank-Walter Steinmeier, who is expected to quit the post early next year as he has been nominated as president.
"In any case, no decision has been made" on who would be the candidate to challenge Merkel, said Sueddeutsche Zeitung.
Merkel on Sunday declared that she will seek a fourth term, while the SPD is due to name its candidate at the end of January.