The 24-year-old, whose Colchester contract was due to run out this summer, has agreed a deal until 2017.
He has scored 14 times in 29 games for the League One strugglers this season, having started the campaign with a career tally of 25 goals.
"I enjoyed my time at Colchester, but my ultimate goal was to move back up the leagues," he told the club website.
"This is a great opportunity to do that with a team heading in the right direction.
"I got the call on Thursday and I was down here that night, got the medical done, and I was signing Friday morning."
Sears will be competing with Daryl Murphy, David McGoldrick and Noel Hunt for a place in Mick McCarthy's side.
The former England Under-21 international began his career with West Ham and was tipped for big things after scoring the winning goal on his debut for the Hammers, aged 18, in a 2-1 Premier League victory over Blackburn in March 2008.
However, he was unable to establish a regular place in the side and had loan spells at Crystal Palace, Coventry and Scunthorpe, before joining Colchester three years ago.
Promotion-chasing Ipswich, third in the Championship, allowed striker Conor Sammon to end his loan spell from Derby this week to join Rotherham.
Colchester, who are 22nd in League One, must now face a relegation battle without their top scorer.
"We have already made inquiries into the availability of a number of players and we will continue to do that," said boss Tony Humes on the search for a replacement.
Ipswich Town have signed Colchester United striker Freddie Sears for an undisclosed fee.