Alex Albon will return to the Formula 1 grid as replacement for Mercedes-bound George Russell in the 2022 season.
Red Bull's current reserve driver, who raced for the team last season, will team up with Canada's Nicholas Latifi, who is confirmed at Williams for a third campaign.
Albon made his debut with what was Toro Rosso, Red Bull's sister team, in 2019 and was promoted to the senior squad after just 12 races with Pierre Gasly moving the other way.
The 25-year-old continued for a full season in 2020 but was replaced by Sergio Perez for this year, dropping down to a test and reserve role.
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"I am really excited and looking forward to returning to a Formula 1 race seat in 2022," said Albon, who twice finished on the podium at Red Bull but failed to consistently race near Max Verstappen.
"When you take a year out of F1 it's never certain you will make a return so I'm extremely thankful to Red Bull and Williams for believing in me and helping me on my journey back to the grid."
Albon was swiftly congratulated on his comeback on social media by Russell, with the two good friends. Russell was finally named as Lewis Hamilton's team-mate for 2022 at Mercedes on Tuesday.
Red Bull boss Christian Horner revealed at last week's Dutch GP that Williams and Alfa Romeo were both interested in signing Albon for next year.
But with Williams running Mercedes engines, and Red Bull becoming an engine supplier from next season with Honda's technology, Toto Wolff said that, while he too wanted to see Albon back on the grid, he wanted Red Bull to cut their ties with the 25-year-old before any such move to Grove happened.
Red Bull released their own statement confirming Albon's Williams deal.
The team clarified that they have released the 25-year-old to drive for Williams next year but retain a relationship with him that includes future options.
"We rate Alex very highly and with our line-up for 2022 fixed, our objective was to help find him a seat in another team where he could gain more F1 mileage and experience and we are delighted to have done that with Williams Racing," said Horner.
"Alex has been a hugely valuable asset in his role as our Test & Reserve Driver this year, helping bring performance from the simulator to the car each weekend, but for his career it was important not to spend another season out of an F1 race seat."
Jost Capito, the chief executive at Williams, said: "Alex is one of the most exciting young talents in motorsport, yet comes with a large amount of F1 experience from his time at Red Bull.
"His multiple podium finishes highlight his speed as a driver, and we know he will immediately feel at home with the team at Grove."
The state of play in 2022's driver market
Albon and Latifi's 2022 deals mean six more seats have been filled on next grid's grid this week.
Only three of F1's 10 teams are yet to officially confirm their line-ups - and two of those, Aston Martin and Haas, are almost-certain to retain their respective existing pairings.
Therefore the only seat left truly up for grabs is at Alfa Romeo.
With Valtteri Bottas now confirmed in one, names in the frame for the second include Alpine-supported Guanyu Zhou, Mercedes-backed Nyck de Vries and the incumbent Antonio Giovinazzi, who is affiliated with Alfa's engine suppliers Ferrari.