Stevenson was under contract until the end of the season, but having started just four games during this campaign, an early release was agreed.
The 31-year-old had a spell with Exeter during the 2008-09 season, before a two-year spell with Harlequins.
He moved back to Belfast in 2011 to join his home province, Ulster.
Lock Stevenson made 89 appearances for the Irish province, scoring one try, which came against Treviso at Kingspan Stadium earlier this season.
"I am extremely proud to have played a part in this Ulster team over the last five seasons. Every time I pulled on the shirt it meant something to me" said Stevenson.
"However, the time is right for me to face a new challenge and I am excited about being part of Exeter Chiefs again.
"I appreciate how Ulster have handled this and I wish them every success for the rest of the season."
Ulster's Operations Director, Bryn Cunningham, commented that Stevenson would be "missed both on and off the pitch".
"Player welfare is crucial for this club and in dealing closely with his agent and Lewis himself, we believe that this outcome will give him the best chance of furthering his rugby career," stated Cunningham.
"It also gives an opportunity during the remainder of the current season for Academy players like John Donnan and Alex Thompson, who have been working with Pete Browne in the 'A' set-up, to move up the pecking order, therefore getting exposure at a higher level, as we continue to develop our young, home-grown indigenous players."
"With the addition of the very promising and exciting Kieran Treadwell coming in next season, plus the current impressive displays and continued improvement of Alan O'Connor, this again bolsters our resources of planning for the future in the second row department."
Second row Lewis Stevenson is to join Exeter Chiefs with immediate effect after being released from his contract with Ulster Rugby by mutual agreement.