The 67-year-old was appointed reserve-team coach in 1991, having earlier been at United as a player during the 1960's. He also played for Dallas Tornadoes and Luton Town before managing the latter club.
Highly regarded for his coaching skills, Ryan was working as assistant manager when he took charge of the Reds' first team for the home match in the 1998/1999 season against Middlesbrough, while Sir Alex Ferguson attended a funeral. The popular Scot has more recently worked closely with fellow countryman Brian McClair in helping to develop the young talent at Carrington.
"I think I have a lot of mixed feelings about it," Ryan told ManUtd.com about moving into retirement. "Generally speaking, a football life is a good life so I'm giving up a lot of enjoyable things.
"I guess it's a wrench but everybody has to retire at some time. I'd like to think I'll still come to Carrington to watch the boys. Of course, you also become friends with the coaches and you don't just stop that friendship."
Everybody at Manchester United would like to wish Jim a happy retirement.