The visitors were a man down in the 13th minute when Fabian Wilnis denied Cameron Jerome a goalscoring chance.
Lewis Price comfortably saved Michael Ricketts penalty, but the striker had his revenge on the half hour when he drilled home Joe Ledley's through ball.
Jimmy Juan's 85th-minute free-kick looked to have snatched a point, but Koumas fired home in the last minute.
Ipswich boss Joe Royle:
"We were terrific with 10 men in the first half. We controlled the game and created some chances. "But I'm disappointed with the sending off. Fabian is adamant there was barely any contact - certainly not enough to put 16 stone on the floor.
"Referees have got to get to know the players who take a dive."
Cardiff: Alexander, Weston, Cox, Loovens, Barker, Koumas, Whitley, Ledley, Parry, Jerome, Ricketts (Lee 88).
Subs Not Used: Ardley, Boland, Mulryne, Margetson.
Booked: Whitley.
Goals: Ricketts 30, Koumas 90.
Ipswich: Price, Sito, Wilnis, De Vos, McEveley (Horlock 15), Magilton (Clarke 65), Richards, Juan, Williams, Forster, Naylor, Clarke (Currie 87).
Subs Not Used: Proudlock, Supple.
Sent Off: Wilnis (13).
Goals: Juan 86.
Att: 8,724
Ref: A Marriner (W Midlands).