Midfielder Alex Gilbey has signed for Championship side Wigan Athletic for an undisclosed fee.
Gilbey, quoted as being an upcoming Premier League prospect, played in over 35 matches for Colchester last season.
Fellow midfielder George Moncur has joined Championship club Barnsley for a reported fee of £500,000.
The 22-year-old, who was linked with Peterborough United, scored 14 times in 51 appearances for the U's.
Goalkeeper Elliot Parish has joined Accrington Stanley following his Colchester release this summer.
The 26-year-old made 28 appearances for the U's last season.
Defender Luke Prosser has joined the U's on a two-year deal following his release from Southend United.
The 28-year-old, who was part of the Shrimper's squad who were promoted to League One in 2015, joins following 164 appearances for the Roots Hall side.