Sargeant will make his F1 debut next year with Williams, replacing Nicholas Latifi after securing enough superlicence points through his rookie Formula 2 season to make the step up.
The team made the announcement in a short social post on its channels on Wednesday with the hashtag #LS2.
Sargeant was able to pick #2 as his number as more than two seasons have passed since it was last used by Stoffel Vandoorne, who raced with it during his stint at McLaren in 2017 and 2018.
Williams team principal Jost Capito revealed over the United States Grand Prix weekend in October that Sargeant was in line for a seat in 2023 so long as he gained the required superlicence points.
Sargeant passed the threshold by taking fourth place in the F2 championship at the Abu Dhabi season finale, leading to his formal confirmation the day after the race.
Williams aided Sargeant's push to get up to speed by fielding him in four FP1 sessions at the end of the season, as well as handing him a full day of running in the post-race test in Abu Dhabi.
Sargeant said after the test in Abu Dhabi that his goal was to "get comfortable in the car and really push the limit" without the pressure to keep to a practice run plan and look after the car for the full-time driver.
"In the FP1s, it's touch and go because you need to get your 100km and you don't have much time," Sargeant said.
"Whereas today, I could really find the limit and get comfortable with it, as well as just really learning how I can change the car balance to my suiting just through the steering wheel.
"That's obviously work in process, it will be for a while. But it's a good understanding of what those tools do."
Sargeant is set to conduct some private test running in an Alpine F1 car over the winter to further his development before getting a day-and-a-half in the 2023 Williams through pre-season testing.