Bottas' future has been a constant source of speculation over the past few months, and despite his desire to remain at Mercedes as team-mate to Lewis Hamilton, the team is set to opt for youth by promoting Russell from Williams.
Bottas replaces fellow Finn Kimi Raikkonen at Alfa Romeo following the 41-year-old's recent announcement he is to retire from F1 at the end of this year.
Expressing his delight with his move, Bottas, who has been handed a multi-year contract, said: "A new chapter in my racing career is opening.
"I’m excited to join Alfa Romeo Racing for 2022 and beyond for what is going to be a new challenge with an iconic manufacturer.
"Alfa Romeo is a brand that needs no introduction, they have written some great pages of Formula 1 history and it’s going to be an honour to represent this marque.
"The potential of the setup in Hinwil is clear and 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.
"I’m grateful for the trust the team has put in me 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.
"I know Fred [Vasseur] well and I am looking forward to getting to know the rest of the team I am going to work with, building relationships as strong as the ones I have at Mercedes."
Welcoming Bottas to a team that has yet to force its way into the F1 spotlight since its rebranding from Sauber in 2019, team principal Fred Vasseur said: "We are looking forward to our journey together.
"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 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 Formula 1. We are excited for what the future holds in store.”
Like Raikkonen before him, Bottas brings a wealth of experience as the 32-year-old has made 169 starts in an F1 career that started with four seasons at Williams prior to a 2017 switch to Mercedes.
Alongside Hamilton, Bottas has helped Mercedes win four consecutive constructors' championships, and with his sights set on a fifth before he leaves at the end of the season.
Bottas, who has so far scored nine wins and 17 poles with Mercedes, added: "I am proud of what I have achieved in Brackley.
"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.”
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