The news ends months of speculation about Williams' driver line-up and was announced alongside news that Robert Kubica, who had been in the running for Sirotkin's seat, will join the team as a reserve and development driver.
The Russian will partner 19-year-old Canadian Lance Stroll, the billionaire's son who made his debut with the former world champions last year, in a lineup that will be the youngest on track this year.
The 33-year-old suffered career-changing injuries during a horrific rally crash in 2011, in which his right arm was partially severed and has not raced...
The Russian beat off tough competition to secure the second seat, Sirotkin was reserve and test driver for Renault the past two years following stints in various junior categories.
Claire Williams, Deputy Team Principal said; "After an extensive driver evaluation process, I am thrilled to have our 2018 line-up finalised, and I'm excited to see what next season brings".
BC-CAR-F1-Williams, 1st Ld-Writethru, 493 Sirotkin chosen ahead of Kubica to drive for Williams in F1Eds: Updates with details on Robert Kubica.
Deputy team principal Claire Williams said the Mercedes-powered team, fifth overall a year ago, were confident the new pairing would deliver.
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"Robert will have a particularly active role within the team this year, including participation in the Barcelona pre-season testing, in-season testing, and will also take part in a number of race weekend practice sessions", a Williams announcement confirmed.
Kubica will get further opportunities to prove to Williams and the wider F1 paddock that he is capable of making a racing return.
"He will be able to assist the race drivers and engineers deliver better qualifying and race performance, and of course, be available to stand-in as the race driver if required".
"My ultimate goal remains to race again in Formula One and this is another important step in that direction: I can not wait to get started".
"It took a huge amount of work to get where I am", Sirotkin said.
The Russian was assessed to have produced the quickest times when adjusted for fuel and tyres and will make his full F1 debut on March 25 at the Australian Grand Prix. He will also support the team's media and sponsorship support programme.
With Felipe Massa once again heading into retirement, Williams were in the hunt for a new driver for this year's championship.