The 25-year-old has long been one of the most coveted frontmen in world football, but Bayern have always been in pole position to sign him.
Reports that a four-year deal with Bayern - to begin next summer - were already in place emerged on Tuesday and after Dortmund's 2-0 extra-time win over TSV 1860 Munich in the DFB-Pokal that evening, he was quizzed by the waiting media.
When asked if he would confirm a move to Bayern in January, Lewandowski told Sport 1: "Yes, because I can officially sign the new contract in January."
He then explained to Sky Deutschland that he believes he could even leave before the summer.
"Right now, I can't say anything," added the Poland international. "Normally, I can only reveal it officially in January.
"At this moment, it is still too early and we'll have to wait a bit. But I think, I can leave in January."