This year, Russell's been competing in Formula 2 - a series he now leads - while being a test and reserve driver at Mercedes-AMG's F1 team.
"It is a huge honour to join a team of Williams' prestige and heritage", Russell said. "From watching the races when I was a child, it feels surreal that I will now be lining up on the grid, alongside..."
"I am incredibly excited to start working with everyone at Grove and to take my first steps as a Formula One race driver. I can not wait for Melbourne next year and to join Williams at the start of this exciting part of their journey".
Williams has struggled for performance over past few seasons and has dropped to new lows this year after scoring just seven points in 16 races.
"We have always tried to promote and develop young talent at Williams, and George fits that ethos perfectly".
She added: "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".
Williams has yet to confirm the identity of Russell's 2019 team-mate, though current racer Sergey Sirotkin and Renault tester Artem Markelov are understood to be in contention for the seat.
Williams have enjoyed a glorious past, winning a combined 16 drivers' and constructors' championships, but they are now in the midst of a miserable campaign which leaves them rooted to the foot of the team standings with only seven points.
Hamilton, who can clinch his fifth world championship in Texas next week, is at Mercedes while Norris, 18, will be making his race debut with McLaren in 2019 after competing against Russell in Formula Two.