The veteran Polish forward, 37, scored 119 goals for Barcelona in 191 games across all competitions since joining from Bayern Munich in 2022.
Lewandowski helped Barca to three La Liga titles, including this season's trophy, and the Copa del Rey in 2025.
"After four years full of challenges and hard work, it's time to move on," said Lewandowski in a post on Instagram.
"I leave with the feeling that the mission is complete. Four seasons, three championships."
The forward joined with Barca at a low ebb and struggling financially, helping restore them to the Spanish throne and also compete for the Champions League.
"Barca is back where it belongs," continued Lewandowski.
"I will never forget the love I received from the fans from my very first days. Catalonia is my place on earth."
Lewandowski will play for the last time at Barcelona's Camp Nou stadium on Sunday against Real Betis.