Lewandowski was expected to leave Dortmund for Bayern Munich in the close-season, but the transfer did not materialise.
The Poland international has been in fine form for Jurgen Klopp's men so far this season, scoring 13 goals in a campaign that appears poised to be his last at Signal Iduna Park.
Bayern, Chelsea, Manchester United and Real Madrid have all been linked with a move for the 25-year-old.
"I will reveal my future next year," he told Sport-Bild. "No one needs to get in touch with me anymore.
"I have decided what I am to do. I simply want to have a new challenge next season, despite the fact I am feeling very, very happy being at Borussia Dortmund."
"There's a clean-cut agreement that Lewandowski will leave in the summer," chief executive Hans-Joachim Watzke said. "We're looking for replacements."
