The 29-year-old agreed the deal after leaving Hearts as part of their restructuring under Ann Budge.
"In January I knew Partick were interested in me so they've showed a lot of loyalty to come back in for me," said Stevenson.
"When I sat back with my family and looked at the options I had in front of me, Partick ticked all the boxes."
Former Ipswich, Ayr and St Johnstone player Stevenson scored eight goals last term as Hearts were relegated from the Scottish Premiership.
He is manager Alan Archibald's first summer signing and his contract has an option for a third year.
"It's a great fan-base and they are lively as well. It's somewhere I think I'm going to enjoy playing my football
"In January I had to be loyal to Hearts, it's a great club that I have a great connection with, and always will have," Stevenson added.
"At that stage I was never going to walk away from it, and part of me was a bit worried Partick wouldn't come back for me.
"But I'm looking forward to getting started now, to getting in and meeting the boys, getting away to pre-season and then getting back into the football.
"I looked at where I enjoyed coming and playing, and outside probably Parkhead, I enjoyed playing here as much as anywhere.
"It's a great fan-base and they are lively as well. It's somewhere I think I'm going to enjoy playing my football.
"And when I spoke to a couple of players who had played here, who had nothing but good things to say about the club as a whole, it became an easy decision."
Thistle recently saw out-of-contract full-back Aaron Taylor-Sinclair join Wigan Athletic, but the Maryhill men have extended the deals of Gary Fraser, Stuart Bannigan, Paul Gallacher, Ryan Scully, and Kris Doolan.
And Archibald, who was delighted to sign someone of Stevenson's experience, is hopeful of bringing in more new players.
"We've got a few targets but all the clubs are after the same players and it's a fight to get them," said the manager.
"We still need a few players in and we're working at it all the time.
"[Ryan] can play a number of positions and he gives us a goal threat as well. He's exactly what we're looking for.
"He's got a real drive. He's ambitious and he wants to do well."
Partick Thistle have signed former Hearts midfielder Ryan Stevenson on a two-year contract.