Ben Youssef, 23, will arrive from CS Sfaxien after competing in this month's 2015 CAF African Nations Cup with his country. He was named 2013's Tunisian player of the year. A club statement read: "FC Metz have completed their second signing of the winter transfer window.
"The club continue to reinforce their attack with the arrival of a new forward: the Tunisian Fakhreddine Ben Youssef has signed a three-and-a-half-year contract with the club."
Metz are above the relegation zone in the French top flight on goal difference alone at the halfway stage of the season, with 18th-placed Evian having a game in hand. Ben Youssef will be tasked with pepping up an attack which has produced only 17 goals in 19 games.