The midfielder becomes Alan Archibald's first capture of the summer as he bids to bolster his squad ahead of the new Premiership campaign.
It has been a productive few weeks for the Jags boss as a number of his Firhill stalwarts, including Paul Gallacher, Stuart Bannigan and Kris Doolan, have put pen-to-paper on fresh contracts with the club.
And Stevenson has now been added to the red and yellow ranks after agreeing a switch that will keep him at Thistle until the summer of 2016, with the option for a further 12 months.
The 29-year-old was one of a number of players Archibald was interested in during the January transfer window but decided to stay at Tynecastle for the second half of the top flight campaign.
Just days after the Jambos' relegation to the Championship was confirmed, Stevenson, who still had a year to run on his Capital contract, quit the club for the second time after refusing to take another pay cut to remain with Robbie Neilson's side.
He has now clinched a move to Firhill as Archibald lands the first of what he hopes will be at least half a dozen additions to his squad this summer.
Having lost Aaron Taylor-Sinclair, allowed Lee Mair to leave at the end of his short term contract and seen loan stars Lyle Taylor, Prince Buaben, Chris Erskine and George Moncur return to their parent clubs, more new faces will arrive at Thistle ahead of the big kick-off.
As revealed in SportTimes last month, a central defender is Archibald's priority this summer while a replacement left-back must also be brought in to replace Taylor-Sinclair, who last week penned a deal with Wigan.
Thistle return to pre-season action on June 27 and will spend a week at a training camp in Spain before facing Dumbarton in their first friendly of the summer as they gear up for their second top flight campaign.