The former Rovers midfielder, one of the greatest players to represent the club, has been out of the game since leaving League Two promotion-chasers Salford City by mutual consent in March.
Wellens, 41, has previously managed Oldham Athletic and Swindon Town.
He led the Robins to the League Two title under the points per game format in 2019-20 before leaving for Salford in November, 2020.
On the appointment, chief executive Gavin Baldwin said: “Having gone through a lengthy process and interviewed potential candidates, the board came to a unanimous decision that Richie was the right man to lead the club forward.
“He knows what this club is about and speaking to him it is clear to see his drive and determination to be successful as our manager.
“I’m sure our fans will join me in welcoming Richie back to Doncaster Rovers for a third spell and give him their full support.”
Wellens has become the permanent successor to Darren Moore, who left the club at the start of March with Andy Butler handed the position on an interim basis.