Fletcher came through the youth ranks of Manchester United and spent two decades at the club.
The former Scotland captain won five Premier League titles, one FA Cup, one UEFA Champions League among numerous other honours in his time at Old Trafford.
Fletcher spent time with West Bromwich Albion and Stoke City during the latter stages of his career before returning to Manchester United as an under-16 coach last October.
The 36-year-old has however been elevated from his previous position and will now be training United’s senior side.
Commenting on the appointment, Fletcher said: "I am delighted to be returning to the club as a first-team coach.
"It's a very exciting time for this young team and I am looking forward to taking the next steps in my coaching career by working with Ole and his staff."
Solskjaer added: "Darren has the United DNA running through his veins and he knows exactly what it takes to be a Manchester United player.
"He is at the start of his coaching career and his experience both on and off the pitch along with his winning mentality, commitment and hard work will be a great addition to the staff."
Fletcher won 80 caps for the Scotland national team, scoring five goals.
