Stevenson, who has represented Ireland at Schools, Under-19 and Under-21 level, was previously on the books at Ulster before joining Exeter Chiefs.
His impressive form over a number of seasons culminated in him gaining selection for the National League 1 Team of the Year in 2009. This form alerted Harlequins who signed him that summer and he has gone on to make 29 appearances for the London outfit.
Stevenson said: "I'm very excited to be joining Ulster. When I left the province it was always my intention to come back with a bit of experience under my belt.
"It's a great time for Ulster Rugby at the moment and it's brilliant to be able to come on board. It's going to be an exciting new challenge for me and I'm delighted."
Stevenson joins as a replacement for Ryan Caldwell who has been released a year early from his contract to join Bath in the summer.