Valtteri Bottas will replace Nico Rosberg at Mercedes, after Williams confirmed the return of Felipe Massa on a one-year deal for the 2017 Formula One season.
Having finally defeated team-mate and long-term rival Lewis Hamilton to reign supreme in F1, Rosberg announced his retirement on December 2 - just five days after winning the title.
Mercedes were consequently left scrambling to find a replacement for a seat they did not expect to be left vacant and Bottas quickly emerged as favourite to fill the void left by Rosberg's departure.
On Monday, Bottas' move to Mercedes was finally set to be rubber-stamped, with Massa having reversed his retirement to plug the gap at Williams left by the Finn's departure.
In a Q&A with the Williams' website, Massa said: "Valtteri has been offered a fantastic opportunity and, as a result, an opportunity arose for me.
"Going to the current world champions is a great opportunity for Valtteri. He is a talented racer and it was great working alongside him for three years. I wish him all the best for the next chapter of his career."
Mercedes, meanwhile, tweeted an image of a jigsaw puzzle featuring Massa and Pascal Wehrlein - whose move to Sauber was confirmed earlier on Monday.
The third person in the puzzle was largely obscured, but part of Bottas' face could be seen and Mercedes subsequently tweeted: "Might take us a while to get to the bo77om of this #F1 mystery..." - a reference to Bottas' race number last season.