The Spaniard has signed a long-term contract starting from next season that will take the Grove-based outfit into the new regulations that will sweep into the sport in 2026.
In a statement on the team's website, Sainz said: “I am very happy to announce that I will be joining Williams Racing from 2025 onwards. 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. The ultimate goal of bringing Williams back to where it belongs, at the front of the grid, is a challenge that I embrace with excitement and positivity.
"I am convinced that this team has all the right ingredients to make history again and starting on January 1 I will give my absolute best to drive Williams forward alongside every single member of the team.
Team principal James Vowles added: "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.
Very happy to announce I'll be joining @WilliamsRacing next season! Excited about the project and the challenges ahead of us! 💪🏻
Muy contento de poder anunciar que el año que viene me uniré a @WilliamsRacing! Entusiasmado con el proyecto y con los retos que tendremos por… pic.twitter.com/oUg5IexunR
"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. 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.
"Their belief in this organisation’s mission demonstrates the magnitude of the work going on behind the scenes. People should be in no doubt about our ambition and momentum as we continue our journey back to competitiveness – we are here, we are serious and with Dorilton’s backing we are investing in what it takes to return to the front of the grid."