George Russell will partner Lewis Hamilton at Mercedes from next season, the team have announced.
The 23-year-old Briton replaces Valtteri Bottas, whose move to Alfa Romeo from 2022 was revealed on Monday.
Russell joins Mercedes after three years at Williams in which he underlined his credentials for a seat in a top team.
Russell said he was "absolutely buzzing" at the move.
"I'm under no illusions as to the scale of the challenge; it's going to be a steep learning curve."
Mercedes team principal Toto Wolff said: "George has been a winner in every racing category - and the past three seasons with Williams have given us a taste of what the future could hold for him in F1."
The move comes with a degree of inevitability about it - Russell has been a Mercedes protege since 2017 and has emerged as one of F1's brightest new stars since he made his debut in 2019.
He has produced a series of standout performances for a team whose car is one of the slowest on the grid, most notably qualifying a remarkable second in the rain at the Belgian Grand Prix last month.
And Russell underlined his credentials for a seat with the world champions when he stood in for Hamilton at last year's Sakhir Grand Prix after the seven-time world champion contracted Covid. Russell was on course to win the race until the team made a mistake at a pit stop.
Wolff, who did not reveal the length of Russell's contract, paid tribute to Bottas' achievements and contribution over his five years at Mercedes and said: "This hasn't an easy process or a straightforward decision for us."
He added that he believed Russell and Hamilton would work well together over the coming years.
"It is our challenge together to help George continue learning within our environment and alongside Lewis, the greatest F1 driver of all time," Wolff said.
"I am confident that, as their relationship grows, they will form a strong team and deliver for Mercedes on and off the track in the years ahead."
Russell said he had "mixed emotions" after three seasons at Williams, a spell he had "loved every moment" of.
But he added: "It's a huge opportunity and one I want to grab with both hands.
"My target must be to reward the trust that Toto, the team, and the board have placed in me by ensuring I play my part in continuing that success and I want to do my new team-mates proud.
"Of course, one of those new team-mates is in my opinion the greatest driver of all time.
"I've looked up to Lewis since I was in go-karts and the opportunity to learn from someone who has become a role model both on and off track can only benefit me as a driver, a professional, and a human being."
Elsewhere in F1, Alpha Tauri announced on Tuesday that they were continuing with Frenchman Pierre Gasly and Japan's Yuki Tsunoda in 2022.
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