The 28-year-old midfielder, a free agent after leaving Bolton in the summer, has agreed a short-term contract until January.
"Ipswich is a sleeping giant in the Championship," Reo-Coker told the club's official website.
"I had a good conversation with the manager (Paul Jewell). He's highly respected in the game for what he has achieved and I know what he's trying to do at the club.
"Ipswich may be at the wrong end of the table at the moment but it's still early in the season and I'm sure with the manager and players the club have, it's only a matter of time before things turn around."
Ipswich are currently second from bottom in the Championship with seven points from 10 games. Reo-Coker could make his debut at Hull next Saturday.