Bottas claimed the last of his nine victories at the Russian GP in September last year — ending a streak of 20 races without a win.
He finished a dismal fifth overall after failing to win a race in 2018 and ended that season with four straight fifth-place finishes.
Bottas is third overall this year but nearly 100 points behind teammate Lewis Hamilton and Red Bull’s Max Verstappen after 13 races.
He will now relaunch his career at a team where he will be under much less pressure.
“A new chapter in my racing career is opening,” Bottas said. “I am relishing the opportunity to help lead the team forward up the grid, especially with the new regulations in 2022 giving the team a chance to make a leap in performance.”
Although he never hit the heights Rosberg did, his relationship with Hamilton was largely a positive one. This proved a welcome relief to Mercedes, after Hamilton and Rosberg fell out and openly feuded in their last two seasons together.
“It has been a privilege and a great sporting challenge to work with Lewis, and the harmony in our relationship played a big part in the constructors’ championships we won,” Bottas said. “I would like to thank (head of motorsport) Toto (Wolff) and the team for the respect in how we took the decision together.”
Wolff praised Bottas for his efforts in helping the team secure four straight drivers’ and constructors’ titles since he joined. Wolff had little time to react when Rosberg stunned F1 by retiring days after his 2016 title, forcing Wolff to move fast to sign Bottas from Williams.
“Valtteri has done a fantastic job and he has made an essential contribution to our success and to our growth,” Wolff said. “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.”
Alfa Romeo team principal Frederic Vasseur knows Bottas well, having worked with him in the F3 and GP3 categories.
“The multi-year deal bringing him to us gives both Valtteri and the team the stability we need to build,” Vasseur said. “We are excited for what the future holds in store.”
Russell has excelled for Williams in a much slower car, showing all his ability with a stunning second place in qualifying at the rain-drenched Belgian GP two races ago. He finished second in that shortened race.
Before that race, Russell had been informed he had a seat at Mercedes in 2022, although he could not yet say so.
He has driven once before for Mercedes.
Russell replaced Hamilton at the Sakhir GP last December when the British driver had the coronavirus, and qualified in an impressive second place behind Bottas despite having little time to adjust to the car.