The Finland international has ended a four-year stay on Teesside, during which he made 92 appearances and scored 18 goals.
Forss spent last season on loan with Bolton in League One, featuring 25 times in all competitions and scoring on five occasions.
A serious hamstring injury picked up after a January midweek game against Port Vale in the Vertu Trophy ended his season prematurely, but he remained with the squad and celebrated promotion on the pitch after victory against Stockport County in the play-off final last weekend.
Forss is expected to be full fit by pre-season and has already started light training duties as part of his rehab,
Dan Barlaser, Alex Gilbert, Darragh Lenihan, Jon McLaughlin and Sammy Silvera have also been released by Boro, with discussions continuing over a new contract for Dael Fry.
Forss, 26, has made nearly 150 appearances at Championship level including spells with Brentford and Hull City, and Steven Schumacher had hoped he could add a touch of versatility and class to the Wanderers attack when he brought him in on deadline day last September.
His fortunes at the Toughsheet were mixed, however, and though he scored a memorable brace in the 4-1 win against Wigan Athletic a few weeks after his arrival, they proved his only league goals for the club.
He scored in the Vertu Trophy against Oldham and Bradford City and his ability to play across the front line was a useful tool but only 13 of his appearances were starts and he struggled to oust Mason Burstow in the central attacking spot for the first few months.
Forss finished the season with 1,153 minutes of football across all competitions and one assist.