The team evaluated Sirotkin alongside Robert Kubica in Abu Dhabi last year as the team evaluated its options for this season.
“To say I’m happy and proud to join such a famous team like Williams is an understatement,” said Sirotkin.
“It took a huge amount of work to get where I am, and I’m really happy and thankful to everyone involved. The result of our combined efforts has helped me achieve my dream, and rest assured the team can rely on me to deliver my best.”
Williams chief technical officer Paddy Lowe said the team has “conducted a rigorous and exhaustive driver evaluation process.”
“Ultimately, Sergey impressed the team with his driving pace and talent, technical feedback and work ethic, both at the factory and trackside in Abu Dhabi,” said Lowe. “We have an exciting lineup for 2018 which we believe will help us to maximise the potential of the FW41,” he added.
Sirotkin first came to prominence in Formula One four years ago when he was linked with a drive at Sauber. The move never came off, though he made a single appearance for the team in the first practice session for Russia’s inaugural Grand Prix in 2014.
In 2016 he joined Renault’s young driver programme and has made a further six appearances in practice sessions over the last two years.
Sirotkin did not have a full-time drive in a series last year but his third-place finishes in the 2015 and 2016 GP2 championships mean he has more than the minimum required 40 superlicence points to step up to F1.
Williams’ announcement means all 20 race seats in the 2018 F1 field have been filled.
View the updated list of 2018 F1 drivers and teams
2018 F1 season
- ‘It doesn’t matter if we start last’: How Red Bull’s junior team aided Honda’s leap forward
- Honda’s jet division helped F1 engineers solve power unit problem
- McLaren Racing losses rise after Honda split
- Ricciardo: Baku “s***show” was Red Bull’s fault
- “Drive to Survive Episode 1: All to Play For” reviewed