Usmagazine.com said the announcement was made during the Television Critics Association press event in Los Angeles Tuesday.
Fox, who is best known for his TV roles in "Family Ties," "Spin City," "Scrubs" and "Boston Legal," went into semi-retirement about a decade ago, shortly after announcing he suffered from Parkinson's disease.
The 47-year-old Canadian-born actor is to play the wheelchair-bound love interest for Tommy Gavin's ex-wife, Janet, on "Rescue Me." Leary plays Gavin and Andrea Roth plays Janet.
"He's a very funny, very original character," Usmagazine.com quoted FX President John Landgraf as saying about the role Fox will play.