N'Guessan will officially join the Reds in January, but he's then spend the rest of the 2026/27 campaign on loan at Ligue 1 side Brest.
The 18-year-old is a product of St-Etienne's highly-rated academy. He made his first team debut in January 2025, and went on to feature 13 times, scoring one goal.
He played seven times for France at the 2025 U20 FIFA World Cup and was named in the team of the tournament at last summer's U17 UEFA European Championship.
The Premier League only permits clubs to loan six players to overseas clubs in a single season and that is currently taken up by Harvey Elliott (Valencia), Lucca Brughmans (Genk), Ifeanyi Ndukwe (Levante), Armin Pecsi (TSV Hartberg), Calvin Ramsay (St Mirren) and Amara Nallo (HJK Helsinki).
Liverpool have an option to cut short Brughmans or Ndukwe's loans in January, which would allow N'Guessand to move onto Brest.