Valtteri Bottas has replaced 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 rubber-stamped, with Massa having reversed his retirement to plug the gap at Williams.
"I have been waiting a long time to drive for a team like this, so I'm really proud," said Bottas in a live video posted on Mercedes' official website.
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."
Prior to confirming Bottas' arrival, Mercedes 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 an updated version with their new driver fully visible.
Bottas joined Williams as a test driver in 2010 and was promoted to a race seat ahead of the 2013 season, securing six podiums in the following year to outscore team-mate Massa by more than 50 points.
Williams struggled last season and Bottas gained his only podium finish at the Canadian Grand Prix to take his career tally in F1 to nine.
Massa had retired last season when Williams announced that 18-year-old rookie Lance Stroll was to take a seat as Bottas' partner.
However, the 35-year-old has been convinced to stay on for the 2017 campaign, with Williams sanctioning Bottas' move on the proviso that a suitable replacement was available.