No details of a fee have been released, although it is believed that Liverpool will receive in the region of one million euros for the 20-year-old.
Mavinga, who spent the second-half of last season on loan at Belgian champions Genk, will only be free to make his Rennes debut after he returns from Colombia, where he will be competing in the under-20 World Cup with France later this month.
Mavinga started his career at Paris Saint-Germain and becomes the Brittany outfit's fourth summer signing following in the footsteps of goalkeeper Benoit Costil, and midfielders Julien Feret and Jonathan Pitroipa.
Rennes, who finished sixth in Ligue 1 last season, have also confirmed that centre-back and captain Kader Mangane, 28, has agreed a one-year contract extension, tying him to the club until June 2014.
What the stars mean:
★ Poor ★ ★ Promising ★★★ Good ★★★★ Very good ★★★★★ Exceptional