The Tractor Boys backed Cook to reshape the squad at Portman Road over the course of the summer transfer window, but results have been mixed.
Ipswich sit in eleventh spot in the League One table and are just seven points off the playoff places, but the club have decided to make a change in the dugout.
The club CEO Mark Ashton said in a statement: “Firstly, we would like to thank Paul and his team for their efforts in what has been a difficult transition period for everyone.
“I know how much Paul wanted to succeed at this club. However, following discussions with the board, we have decided a change is needed for our long-term interests.
“At this moment, there is no one lined up to replace Paul. There will be an extensive search to find the best replacement for the club.
“I will communicate more to the media and our supporters on Monday.”
The Tractor Boys have the ninth best record in League One taken over the last 12 games, having picked up as many points as Sunderland and just one fewer than Portsmouth.
Ipswich are due to lock horns with Charlton Athletic in an away League One fixture on Tuesday night as they start life without the experienced Cook.
