LLOYD James has completed his free transfer move to Colchester, after being released by Saints earlier this summer.
The 22-year-old has been training with the League One side in recent weeks and was offered a contract a couple of days ago after impressing.
James, who can play in midfield as well as at right-back, was one of the highest profile casualties on Saints' released list in May.
But he is back on his feet now and will return to St Mary's when Saints host the U's on Saturday, September 18.
Colchester are yet to announce how long the deal is for.
Manager John Ward said: "I'm really pleased that we have been able to bring Lloyd in, he has a good pedigree for a 22-year-old and has already made around 70 appearances for Southampton.
"He has a great range of passing, being accurate with short and long passes with either foot, and has worked his way into the squad over the past three weeks.
"Having the right people here as well as them being good players is important to me and Lloyd has fitted in well with the squad socially and has earnt their respect with his performance in training and the games."