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.

In a series of tests, researcher Ann Smet, of the University of St Andrews, offered the animals a choice between two identical buckets, then pointed at the one containing a hidden treat.
From the first trial, the elephants chose the correct bucket.
The results are published in the journal Current Biology.
The scientists worked with captive elephants at a lodge in Zimbabwe.
Prof Richard Byrne, a co-author on the research, said the elephants had been rescued from culling operations and trained for riding.
"They specifically train the elephants to respond to vocal cues. They don't use any gestures at all," said Prof Byrne.
"The idea is that the handler can walk behind the elephant and just tell it what to do with words."
Mud bath: Why elephants enjoy a good wallow
Despite this, the animals seemed to grasp the meaning of pointing from the outset. This makes them the only non-human animals to understand the gesture without being trained to do so.
In previous studies, Prof Byrne said, our closest primate cousins, the chimpanzees, proved to be "hopeless" at at similar task.
Ms Smet added that she had been impressed by the animals' apparently innate understanding of the gesture.
"Of course we had hoped that the elephants would be able to learn to follow human pointing, or we wouldn't have done the experiment in the first place," she said.
"But it was really surprising that they didn't seem to have to learn anything.
"It seems that understanding pointing is an ability elephants just possess naturally and they are cognitively much more like us than has been realised."
Source: BBC Nature
Watch how elephants use mud as a sunblock
Prof Byrne said studying elephants helped build a map of part of the evolutionary tree that is very distant from humans.
"They're so unrelated to us," he told BBC News. "So if we find human-like abilities in an animal like an elephant, that hasn't shared a common ancestor with people for more than 100 million years , we can be pretty sure that it's evolved completely separately, by what's called convergent evolution."
The researchers said their findings might explain how elephants have successfully been tamed and have "historically had a close bond with humans, in spite of being potentially dangerous and unmanageable due to their great size".
But the scientists added the results could be a hint that the animals gesture to one another in the wild with their "highly controllable trunks".
Ms Smet told BBC News: "The next step [in our research] is to test whether when an elephant extends its trunk upwards and outwards - as they regularly do, such as when detecting a predator, this functions as a point."
Ipswich Town have signed Colchester United striker Freddie Sears for an undisclosed fee.
[NEXT_CONCEPT]
African elephants have demonstrated what appears to be an instinctive understanding of human gestures, according to UK scientists.