Valtteri Bottas is to move to Alfa Romeo from Mercedes next season.
The announcement of the Finn's switch paves the way for Mercedes to declare the signing of George Russell to partner Lewis Hamilton from 2022.
Bottas said: "I am proud of what I achieved at Mercedes and I am fully focused on finishing the job as we fight for another world championship.
"But I am also looking forward to the new challenges that await me next year with an iconic manufacturer."
Bottas joins Alfa Romeo as an effective replacement for Kimi Raikkonen, who is retiring from Formula 1 at the end of the season.
The news is the latest in a choreographed series of announcements, with Mercedes expected to confirm the signing of Russell in the coming hours or days.
Bottas has won nine races in five years since he joined Mercedes in 2017 as a replacement for German Nico Rosberg, who retired after winning the 2016 world championship.
The 32-year-old has proved a capable back-up to Hamilton but has not been able to consistently challenge the seven-time champion.
Hamilton said Bottas had "been the best team-mate I've had the pleasure of working with".
Alfa Romeo said Bottas had signed a "multi-year deal", which is likely to be for at least two or three years.
Bottas said he believed that the Alfa Romeo/Sauber facilities in Hinwil in Switzerland had "clear potential" and he hoped the new technical regulations being introduced by F1 in 2022 would give "the team a chance to make a leap in performance".
Alfa Romeo are ninth in the constructors' championship after 13 races this season.
"I'm grateful for the trust the team has put in me," Bottas said, "and I cannot wait to repay their faith: I'm as hungry as ever to race for results and, when the time comes, for wins."
Alfa Romeo team principal Frederic Vasseur, who ran Bottas in junior categories, said: "With him, we bring to Hinwil a strong team player with experience at the sharp end of the grid.
"Valtteri has been an integral part of a team that rewrote the history books and he has four constructors' world titles to his name: he is the right driver to help Alfa Romeo Racing make a step forward towards the front of the grid.
"Our relationship goes back a long way, to when we worked together in successful campaigns in F3 and GP3, and his talent and skills have been evident ever since, only growing with the time. I'm really looking forward to seeing him put his ability to work for the good of the team.
"The multi-year deal bringing him to us gives both Valtteri and the team the stability we need to build our project at a crucial time for F1. We are excited for what the future holds in store."
Alfa Romeo have made no announcement on a second driver.
They are believed to be considering options that include their current driver Antonio Giovinazzi, former Red Bull driver Alexander Albon and Formula E world champion Nyck de Vries.
Mercedes team principal Toto Wolff said: "This hasn't been an easy process or a straightforward decision for us. Valtteri has done a fantastic job over the past five seasons and he has made an essential contribution to our success and to our growth.
"Together with Lewis, he has built a benchmark partnership between two team-mates in the sport, and that has been a valuable weapon in our championship battles and pushed us to achieve unprecedented success.
"He would absolutely have deserved to stay with the team, and I am pleased that he has been able to choose an exciting challenge with Alfa next year to continue his career at the top level of the sport; when the time comes, he will leave us with huge goodwill from every single member of the team."
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