Gasly's team-mate at Alpha Tauri is Yuki Tsunoda
The 26-year-old has 16 points from nine races this season, with his highest finish coming when he placed fifth at the Azerbaijan Grand Prix.
"I have been with this team for five years now," said Gasly, who won his maiden grand prix in Italy in 2020.
"I am proud of the journey we've been through together and the progress we have made.
"This year's new regulations have created new challenges for us and being able to plan our development with the team for the next 18 months is a good working basis for the future."
Gasly was under contract as a Red Bull driver until 2023, but he said last month he was staying at Alpha Tauri - the drinks brand's second team - next season.
At last weekend's Canadian Grand Prix, Alpha Tauri team principal Franz Tost said this was "100% confirmed".
A spokesperson for Alpha Tauri said the formal announcement "stops speculation".
Gasly had a brief spell away from the Alpha Tauri team, who were formerly known as Toro Rosso, when he was promoted to the senior Red Bull outfit in 2019.
However, he struggled alongside Max Verstappen and was demoted back to Toro Rosso after half a season, in a swap with Alexander Albon.
"He is definitely in the group of the best and most competitive drivers in F1 and has proven his abilities during all the time he has spent with us," said Alpha Tauri team principal Franz Tost.
"Undoubtedly, Pierre can play a major role in the team having a successful season next year and it will be down to us to provide him with a competitive car, so that he can continue to deliver excellent results."
-- Courtesy of BBC Sport