Ryan has put pen to paper on a one-year deal with the option for an extension and becomes the Spireites' third new arrival this summer following goalkeeper Aaron Chapman and striker Eoin Doyle to the Proact Stadium.
The 24-year-old, who played alongside Chesterfield assistant manager Leam Richardson at Accrington, told the club's official website: "I wanted to come here because I knew the staff and I knew the football that they wanted to play.
"I admired them when I played here with Accrington. The club sold itself because you just look around and you know it's going places so I wanted to be a part of that."
Richardson added: "Jimmy is someone we identified quite early on who we wanted to bring to the club. He's a fantastic footballer and a great lad. He handles the ball very well and to add him to what we're trying to do at Chesterfield just seems right."
Former Liverpool trainee Ryan, who won the FA Youth Cup with the Merseysiders in 2006 and 2007, joined Accrington in August 2008 following a short spell at Shrewsbury. He made 129 appearances and scored 22 goals in three years at the Crown Ground.
Ryan was restricted to just 27 outings during an injury-hit first season at Scunthorpe but went on to play 49 games in all competitions for the Iron last season.
The Irishman has scored twice against Chesterfield - for Accrington in a 1-1 draw in November 2008 and for Scunthorpe in a 2-2 draw on New Year's Eve 2011.
- Sports & Recreation
- Leam Richardson
- Eoin Doyle