Having previously driven for Toro Rosso, Renault and McLaren, Sainz had been linked with a number of seats for the 2025 season including Alpine, Sauber and Mercedes.
But his immediate future was decided on Monday when it was confirmed he has agreed an initial two-year deal with Williams.
“I am very happy to announce that I will be joining Williams Racing from 2025 onwards,” Sainz said.
“It is no secret that this year’s driver market has been exceptionally complex for various reasons and that it has taken me some time to announce my decision.
“However, I am fully confident that Williams is the right place for me to continue my F1 journey and I am extremely proud of joining such a historic and successful team, where many of my childhood heroes drove in the past and made their mark on our sport.”
Williams team principal James Vowles said: “Carlos joining Williams is a strong statement of intent from both parties.
“Carlos has demonstrated time and again that he is one of the most talented drivers on the grid, with race-winning pedigree, and this underlines the upwards trajectory we are on.
“Carlos brings not just experience and performance, but also a fierce drive to extract every millisecond out of the team and car; the fit is perfect.”
Sainz replaces Logan Sargeant and will race alongside Alex Albon at Williams.
“In Alex and Carlos we will have one of the most formidable driver line-ups on the grid and with huge experience to guide us into the new regulations in 2026,” Vowles added.
