Valtteri Bottas will return to Mercedes as a reserve driver next year after missing out on a seat in Formula 1 in 2025.
Bottas was Lewis Hamilton's teammate at Mercedes from 2017 to 2021 before moving to Sauber when he was replaced by George Russell in 2022.
Following three tough seasons at Sauber his contract was not renewed for 2025 after the Swiss team, which is set to become Audi in 2026, signed Nico Hülkenberg and Formula 2 champion Gabriel Bortoleto for 2025.
Following Hamilton's decision to leave for Ferrari in 2025, Mercedes' driver line-up will feature Russell and rookie Andrea Kimi Antonelli next year, with Bottas acting as reserve.
When it became clear Bottas would not drive for Sauber next year, he started a social media campaign with the phrase "What's next?," which he also used to sell merchandise for a charitable cause.
"I'm happy to finally answer the question I've posed over the past month," Bottas said. "Returning home to the Mercedes family as third driver for 2025 is what's next and I couldn't be more pleased. I want to thank Toto, the team at Brackley, and everyone at the three-pointed star for welcoming me back with open arms.
"Despite the challenges of the past few years, I know that I've still got so much more to contribute to F1. Since I was a five-year-old kid growing up in Nastola, Finland, my focus has been on achieving success in the top tier of motorsport. I've been fortunate to have enjoyed many incredible moments in my 12 years of racing in F1 so far.
"As I return to the place where so many of those moments were achieved, I'm looking forward to using all the knowledge I've gained to help the team to perform and progress towards our goal of fighting for world championships."
Team principal Toto Wolff added: "Welcome home, Valtteri! It's great to have you back.
"His impact and contribution in the five years he was with us previously as our race driver was immense. Along with scoring multiple Grands Prix wins, he played a vital role in five of our championship victories.
"His technical feedback and input were important in helping us to those successes and pushing the team forward. Not only that, but he was also a fantastic colleague and team member. His dry sense of humour and personable nature made him a firm favourite with everyone at Brackley and Brixworth.
"In his role as reserve driver, all those qualities will be incredibly important in helping us compete for world championships and supporting both George and Kimi on track."