Williams have handed Robert Kubica a reserve and development driver position after announcing the signing of Sergey Sirotkin to partner Lance Stroll.
Sirotkin's arrival completes the full driver line-up for the 2018 Formula One world championship, while Kubica's role comes after the Pole suffered serious injuries in a rally crash in 2011.
Kubica had been thought to be in contention for a spot on the grid with Williams and he still hopes to one day make an F1 return.
He said: "I feel in the best physical shape that I have ever been, but it has taken a lot of work to get to where I am now, so I'd like to thank Williams for the opportunities they have given me so far, and for putting their faith in me with this appointment.
"My ultimate goal remains to race again in Formula One and this is another important step in that direction: I cannot wait to get started."
Williams said the 33-year-old "will have a particularly active role" which will see him involved in pre-season testing in Barcelona as well as several race weekend practice sessions.
Sirotkin, 22, will embark on his debut season in the sport, having also fought off reported competition from Paul di Resta.
"To say I'm happy and proud to join such a famous team like Williams is an understatement," said Sirotkin, who replaces retiring veteran Felipe Massa.
"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."
Sirotkin impressed his new employers during a tyre test in Abu Dhabi in November and will debut at the Australian Grand Prix
It is a young team that Williams will put on the track this term, with Stroll only 19.
The Canadian finished 12th last year, his return of 40 points leaving him only three shy of Massa's tally.