Giroud, 34, had been expected to potentially leave Chelsea this summer with his contract set to expire at the end of the season.
However, Chelsea have revealed that the World Cup winner has already put pen to paper, with a new deal being agreed back in April.
Chelsea director Marina Granovskaia said: “Olivier has been such a crucial part of so many big moments for this club, from his contribution to our FA Cup victory in 2018 to scoring 11 goals on the way to winning the Europa League in 2019.
“He has continued to score significant goals, and we will never forget his effort against Atletico Madrid on the way to winning the Champions League this season.
“With so much to still to play for, there was only one decision we could make when we exercised the option to extend his contract back in April.”
While Giroud has extended his stay in west London, Chelsea are still thought to be targeting a new striker in the summer transfer window, with Erling Haaland and Harry Kane on their wishlist.