According to Entertainment Weekly, the 55-year-old actor will play the role of Dr Daniel Charles, who is head of psychiatry.
'Chicago Med' is the second spinoff series to come from 'Chicago Fire', which will be introduced in the 19th episode of the current season, due to air this spring.
Platt follows previous cast additions, including 'The Walking Dead' star Laurie Holden and former 'Law & Order' actress S Epatha Merkerson.
The new spin-off is being penned by 'Chicago Fire' creators Derek Haas and Michael Brandt.
'Chicago PD' was the first 'Chicago Fire' spin-off, which premiered midseason in January 2014. It was recently renewed for a third season.