The 21-year-old, who is a Spain Under-21 international, joined the Stadium of Light from Sochaux's academy back in July 2023 and leaves having made 72 appearances for the club, contributing 12 goals and six assists across his time on Wearside. Sochaux are understood to retain a sizeable sell-on clause from that original deal.
Mayenda was a key figure in Sunderland's promotion from the Championship in 2024-25, scoring the winner in the first leg of the play-off semi-final against Coventry City before netting the equaliser in the play-off final win over Sheffield United at Wembley, a result that sealed the club's return to the Premier League.
He went on to make 20 top-flight appearances last season, scoring twice as Sunderland adjusted to life back among the elite, though he found regular starts hard to come by under manager Regis Le Bris. Despite the reduced game time, Mayenda remained popular with team-mates, staff and supporters, having overcome a difficult first year on Wearside, which included a hamstring injury and a loan spell at Hibernian, to establish himself as one of the club's most promising attacking talents.
Mayenda has signed a five-year contract at Rennes, where he will work under head coach Franck Haise and is set to be deployed primarily on the right wing. The move also returns him to Ligue 1 football, with Rennes preparing for a UEFA Europa League campaign next season.
The sale brings a welcome boost to Sunderland's transfer kitty as they continue to operate within Premier League spending rules, with the funds expected to help fund further additions to Le Bris' squad this summer.
For Rennes, Mayenda's arrival forms part of a busy summer of recruitment, with the Ligue 1 club also working on a separate deal to sign Toulouse centre-back Charlie Cresswell as they strengthen across multiple areas of the squad ahead of the new season.