The 37-year-old had been a sensational signing for the LaLiga side, scoring 120 goals in his 193 games across all competitions.
Despite his record, Barcelona decided not to renew his contract, and Lewandowski will leave as a free agent at the end of June.
Since his exit was confirmed, he had been heavily linked with a move to Italy, with both AC Milan and Juventus said to be keen.
Per The Athletic, Lewandowski has now agreed to join Chicago Fire after visiting the city earlier this month.
“We’re trying to recruit world-class players,” Fire director of football and head coach Gregg Berhalter told Up & Adams earlier this month.
“We think (Lewandowski would) be a great signing not only for the Chicago Fire, but for the league, to have a player of that calibre.
“We see him right up there with Messi in terms of ability, and it’d be great for the city of Chicago.”