The 35-year-old only joined Jose Mourinho's side as a free agent last summer, having departed Manchester United upon the expiration of his contract.
Matic came up with two goals and three assists in 50 games for Roma during his first and only season at the club, but the Giallorossi have strengthened in midfield this summer with the signing of Houssem Aouar and are close to sealing a loan move for Paris Saint-Germain's Renato Sanches.
As a result, Matic was given the green light to leave Roma despite his crucial contributions last season, and he has agreed to a two-year deal with Rennes until the summer of 2025.
Upon his arrival at Roazhon Park, Matic told the official Rennes website: "I feel welcome here. I was very well received. The project that was offered to me pleased me from the first discussions. I know that there are talented players in the squad, I can't wait to get started."
Chief Executive Officer Olivier Cloarec added: "A world-class player is joining us. It's an arrival that confirms the attractiveness of Stade Rennais FC and its ambition.
"Nemanja is a benchmark midfielder in his position. With nearly 600 matches at the highest level, his extraordinary European experience will reflect on the whole club and especially among our young players. The enthusiasm is great at the idea of seeing him walk the lawn of Roazhon Park."
Finally, technical director Florian Maurice likened Matic to his fellow veteran Steve Mandanda, hailing the 35-year-old's "humble but ambitious" demeanour.
"I am very satisfied to welcome Nemanja to our squad. This is the profile we were looking for to support the team in its progress. He has played in the biggest European clubs and his experience is incredible," Maurice said.
"Nemanja Matic is a complete, intelligent midfielder with a strong tactical culture and capable of mastering the tempos and rhythm of a match. Large in size, it will also be important in the aerial field. Like Steve Mandanda, he is a humble but ambitious person who will speak well to our young players.
"We have been talking to him for more than a month and he knows the club thanks to its visibility on the European scene and its performance in Ligue 1. His arrival also confirms the attractiveness of Stade Rennais FC on the international scene."
Matic - who played the full 120 minutes in last year's Europa League final defeat to Sevilla - follows the likes of Roger Ibanez, Justin Kluivert, Cristian Volpato and Benjamin Tahirovic out of the Roma exit door this summer, and he could make his Rennes debut against Lens on Sunday night.