The 29-year-old will initially join the Eastern Conference side for four months, but there is the option to extend his contract to two years should Sinama-Pongolle impress.
Chicago Fire Technical Director Brian Bliss revealed last month that Sinama-Pongolle had been training with the club.
"Florent's a guy with a good resume who is currently without a team," Bliss told the club's official website at the time.
"He'll be getting a look with us in training over the next week as we explore roster development opportunities going forward."
Sinama-Pongolle had been without a club since leaving FC Rostov at the end of last season.