The 33-year-old former Stevenage and Fleetwood player made 52 league starts for Cambridge over the past two years.
He has signed a one-year contract with Mark Cooper's side.
"I was involved in two successful promotion campaigns [with Stevenage and Fleetwood]. I'd love to have a third," Roberts told BBC Radio Gloucestershire.
Speaking of the former Crewe youngster, who has also played for Accrington, Rovers boss Cooper added: "He is a good, solid defender and he'll give our younger players that little bit of guidance."
Cambridge chief executive Jez George told their club website: "Since arriving at the club, Mark has been an absolute professional, on and off the pitch."
On Wednesday, Forest Green confirmed the departure of midfielder Rob Sinclair by mutual consent.
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Forest Green Rovers have signed defender Mark Roberts after his contract at Cambridge United was cancelled by mutual consent.