Przybylko, 28, will be under contract with Chicago through the 2023 season, with the GAM split evenly across the next two seasons.
"We're very pleased to add a player of Kacper's caliber to our roster," Fire sporting director Georg Heitz said in a news release Saturday. "Going into the offseason, we wanted to sign a top attacking player with MLS experience and Kacper has been one of the most effective goal-scorers in this league over the last three seasons.
"With his physical attributes and knack for scoring goals, we believe that he will be a great addition as we continue to build a talented roster for 2022 and beyond. We look forward to welcoming him and his family to the Club."
Przybylko led Philadelphia with 12 goals and added four assists in 34 games (29 starts) in 2021. He had 35 goals and 14 assists in 83 matches (77 starts) over three seasons with the Union and helped the club win its first Supporters' Shield in 2020.
"We want to thank Kacper for his efforts on and off the field with the Philadelphia Union," Union sporting director Ernst Tanner said in a release. "He has played a major role in some of the Union's most memorable moments over the last few years and we wish him all the best as we continue to look to strengthen and add depth to our roster in the coming weeks."
Przybylko competed professionally in his native Germany from 2011-18 before joining Philadelphia.
He has a U.S. Green Card and will not occupy an international slot on the Fire's roster.