The 20-year-old Ghana international completed a deadline-day transfer from the French club on a contract until 2027.
He becomes the fourth signing for Nathan Jones’s squad in the January window, following the arrivals of Mislav Orši?, Charly Alcaraz and James Bree.
Capped 15 times for his country, including two appearances at the 2022 World Cup, Sulemana is already experienced at both club and international level and is comfortable either in a wide or central position.
Having played youth football in his home country, he moved to Denmark to join FC Nordsjælland in 2020, making an immediate impact as he scored 14 times in 42 appearances. That led to Rennes signing him in 2021, with Sulemana continuing to enhance his reputation there.
He departs having made 47 appearances and contributing six goals for the Ligue 1 side, where he has predominantly played out wide.
Southampton manager Nathan Jones said: “This is another really positive addition for us this month. We’ve obviously been keen to add a little more potency to the side, and Kamaldeen is someone who does that for us.
“He’s at a stage in his career where we feel he can help us in the immediate term, but also continue to grow and develop further into the future.”
The highly-rated Sulemana joins Southampton after struggling to make much of an impact in Ligue 1 this season, with just two of his 14 appearances coming as a starter.