Guendouzi, 26, began his career at Lorient before moving to Arsenal in 2018. After loan spells at Hertha Berlin and Marseille, he joined Marseille permanently in 2022 and signed for Lazio in 2024.
Fenerbahce said on their website that they had signed Guendouzi on a four-and-a-half-year deal.
The Frenchman will be known to Premier League fans for his time with Arsenal, where he made 82 appearances.