Devitt, released by Hull at the end of last season, has put pen to paper on a six-month deal at the Proact Stadium and becomes manager Paul Cook's eighth new signing of the summer.
The 23-year-old has impressed throughout Chesterfield's pre-season programme and scored with a superb free-kick against Huddersfield last week to win the game for the Spireites.
"Paul has known Jamie for a long time and his ability has never been in question," said chief executive Chris Turner.
"We've got a lot of talented footballers and Jamie certainly falls into that category,
"Jamie is in the Sammy Morsy, Gary Roberts, Jimmy Ryan bracket and he's been brought in to compete for his place and provide competition for others."
On the six-month deal, Turner added: "We just want to give him an incentive to get into the team, to establish himself in the squad and it may turn into a longer term contract within weeks or months."