Bottas has raced for Mercedes since 2017, winning nine races across that period and helping the Silver Arrows to four constructors' championships.
However, the 32-year-old's future has been the subject of much speculation in recent months and it has now been confirmed he will leave Mercedes at the end of the 2021 season.
Bottas will bring plenty of experience to Alfa Romeo, having twice finished as runner-up to Hamilton in the Driver Standings and also secured 54 podiums in 92 starts for Mercedes.
"A new chapter in my racing career is opening," he said in a statement on Monday.
"I'm excited to join Alfa Romeo Racing ORLEN 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.
"I am proud of what I have achieved in Brackley 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."
The news of Bottas' arrival comes after Raikkonen announced last week he is to retire at the end of the season. Alfa Romeo have yet to announce who will partner Bottas next year.
Bottas will reunite with Alfa Romeo team principal Vasseur after the pair previously worked together at ART in F3 in 2009 and 2010, and in GP3 in 2011.
Vasseur said: "It is a pleasure to welcome Valtteri to the team and 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 Racing ORLEN make a step forward towards the front of the grid."