The 28-year-old Frenchman will replace his countryman Thomas Heurtel who has been at odds with Barca recently. Westermann will re-unite with his former coach Sarunas Jasikevicius.
He played under Saras in Zalgiris during the 2016-2017 and 2018-2019 seasons, spending a season with CSKA Moscow in between.
According to the official press release, Barcelona wanted to sign Westermann all the way back in 2014 but he chose to play for Limoges in his native France instead.
This season, Westermann is averaging 4.8 points, 1.7 rebounds and 2.3 assists in 12 minutes per game. He played nine EuroLeague games for Fenerbahce, starting three of them.