The 20-year-old Briton will make his F1 race debut at the Australian Grand Prix and joins Williams with an impressive racing résumé.
Following a successful start to his career in karting, Russell stepped in to single seater racing in 2014, winning the BRDC Formula 4 Championship at his first attempt as well as becoming the McLaren Autosport BRDC Award winner.
He progressed to the Formula 3 European Championship for two seasons, and then went onto win the GP3 championship in his rookie 2017 season with two races to spare.
He stepped up to the FIA Formula 2 Championship this year, which he is currently leading, whilst also fulfilling a test and reserve Driver role for Mercedes.
"It is a huge honour to join a team of Williams' prestige and heritage," said Russell. "Formula One has been a life-long dream. From watching the races when I was a child, it feels surreal that I will now be lining up on the grid, alongside drivers whom I have admired for years. I would like to thank Claire and everyone at Williams for their faith in making this decision, as well as Toto and the team at Mercedes for their fantastic support throughout GP3 and Formula 2.
"I am incredibly excited to start working with everyone at Grove and to take my first steps as a Formula One race driver. I cannot wait for Melbourne next year and to join Williams at the start of what we all believe will be the start of an exciting new journey."
"I am delighted to announce that George Russell will be joining Williams from the 2019 season," added Claire Williams. "We have always tried to promote and develop young talent at Williams, and George fits that ethos perfectly. He is already highly regarded in the paddock and a driver whose career we have been watching for a while.
"In the time we have spent with him so far, we believe that he will be a great fit for our team; his commitment, passion and dedication is exactly what we need to drive the positive momentum building at Grove as we focus on the future. We are extremely excited to welcome George and work with him moving forward."
Russell, who became part of Mercedes junior driver programme in January 2017, subsequently drove both days of the post-Hungary GP test for the German team, making his F1 GP weekend debut in Brazil with Force India and subsequently participating in FP1 in Abu Dhabi with the Silverstone-based outfit.