Nkounkou has agreed a three-year deal with Everton following the expiration of his contract at Marseille.
The 19-year-old played 14 times for Marseille's B team in the National 2 in 2018-19, France's fourth tier, while also being named on the bench for the first team twice.
He was a rumoured target for Serie A champions Juve but will now be working under Carlo Ancelotti at Goodison Park, though Nkounkou will first bolster David Unsworth's U23 side.
Everton, which is unbeaten since the resumption of the Premier League, has Lucas Digne and Leighton Baines as its options at left-back.
Baines, however, is yet to agree to a contract extension, with the 35-year-old's current deal set to run out at the end of the season.