Lewandowski joined the Spanish giants from Bayern Munich in 2022 and has scored 119 goals in 191 games for Barça.
"I will never forget the love I received from the fans from my very first day," the 37-year-old writes.
The Pole had his big breakthrough at Borussia Dortmund before moving to German rivals Bayern Munich, where he scored 344 goals in 375 games.
It is not known what his next career step will be.
