The veteran striker opted against a contract extension with the LaLiga champions with the club having a clause in place to retain him for the 2026/27 season.
The 37-year-old remained ruthlessly efficient in his four seasons in Spain, with an incredible return of 120 goals scored in 193 games, alongside winning three Spanish titles and the 2025 Copa del Rey.
He now prepares for a new challenge in MLS, after signing a two-year deal in the USA, and he admitted staying in Europe was not an option due to his love for Barca.
"I didn't want to play for any other team in Europe than Barca. I couldn't imagine it.
"It's been a difficult decision for me and my family, but I'm very happy to join this team. I'm looking to win titles in this new stage of my career and life.
Lewandowski has joined Chicago Fire as one of their International Designated Players, and with the 2026 MLS season due to restart in the coming weeks - after the 2026 FIFA World Cup - he could make his club debut against Inter Miami on July 22.