The 22-year-old product of the Glenn Hoddle Academy in Spain has agreed a 12-month contract with an option for a further year.
"AJ will give us competition for places in both wide areas, and offer something different to what we have in the squad," said manager Jez George.
"We feel has the potential to be a real asset for this club in the long term."
Moke made 28 appearances for York last season as they completed an FA Trophy and promotion double, but 17 of them were from the bench and he rejected a new deal offered by the club.
However, he admitted last week he was in two minds over his future, telling BBC Radio York: "I'm really conflicted because I want to play as much as possible and part of me is worried now we're in the Football League I won't get as much game time, but another part of me really wants to prove myself."
Despite that, he has now opted to return to the Blue Square Bet Premier in the hope of helping Cambridge regain the Football League status they lost in 2005.
"It has taken a little longer than we originally anticipated to finalise everything, but we are really pleased to have secured his signature.
"He is a very quick, explosive and direct player who loves dribbling with the ball and we think that he will excite our fans," said George.
Source: BBC Sport