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.
|I´m squeezing this orange until there´s no juice left – LeBron|
|LeBron James leads Cavs past Celtics and into NBA Finals|
|Kyle Busch dominates to win Coca-Cola 600|
|Henderson still confident of Liverpool success|
|Sporting Kansas City 0 Columbus Crew 0: 10-man visitors battle to MLS draw|
|Inglis to captain new-look Maroons, Boyd and Scott axed for Origin I|
|Neymar labels Real Madrid links ´nonsense´|
|Koepka ready to turn seconds into wins|
|Rose ´really proud´ of Fort Worth Invitational win|
|Paul a ´game-time decision´ for series decider, says Rockets coach D´Antoni|
|Egypt president wishes Salah speedy recovery|
|Bonucci: Balotelli has matured|
|Ranieri backs Sarri to succeed at Chelsea|
|Konta admits to doubts after another early Roland Garros exit|
|Warriors´ Iguodala, Looney listed as questionable for game seven|
|Zverev batters Berankis, Goffin back from the brink in Paris|
|Rose cruises to victory at Colonial|
|Neymar: I´m not at 100 per cent yet|
|Toulouse confirm Ligue 1 survival with win over Ajaccio|
|I know what I´ve achieved - Bale not worried about plaudits as he looks to the future|
|Ostapenko ´angry´ after Roland Garros exit, Venus Williams falls at first hurdle|
|Barcelona B relegated to Spain´s third tier|
|Rayo Vallecano win promotion to LaLiga|
|Tarkowski leaves England camp for hernia surgery|
|Ramos ´quite clever´ but Salah injury not to blame for final defeat, insists Robertson|
|WATCH: Ricciardo takes wide receiver role on yacht for Tom Brady pass|
|Until next year – Ronaldo drops new future hint as Real Madrid celebrate with fans|
|Froome stunned by Giro triumph|
|The Triple Crown: Froome´s Grand Tour glory in numbers|
|Hannover sign Wimmer on season-long loan|
|Inter powerless to stop Icardi exit, admits Spalletti|
|Ashton makes England statement with Barbarians hat-trick|
|Watson masterclass seals third IPL title for Super Kings|
|Kozlova ends Ostapenko´s Roland Garros reign on day one|
|Imperious Zverev breezes into second round|
|Froome wins Giro d´Italia to complete Triple Crown|
|Stricken Salah confident over World Cup hopes|
|Verstappen highlights change in approach amid Rosberg criticism|
|Strinic claims three-year deal at Milan|
|Napoli captain Hamsik considering China offer|
|Chinese outsider Wang shocks Venus Williams in Paris|
|Redemption two years in the making - Ricciardo revels in Monaco glory|
|Flawless Molinari holds off McIlroy at Wentworth|
|Karius ´infinitely sorry´ but thanks Liverpool fans for support|
|Root and Bayliss dismiss ´outrageous´ fixing allegations|
|Giggs: Bale the best Welsh player I´ve seen|
|Ricciardo ends Monaco hoodoo despite mechanical issue|
|Lyon star Fekir denies being close to Liverpool switch|
|Pogba: I learned a lot from Abou Diaby|
|Where next for Real Madrid stars Ronaldo and Bale?|
|Shingler and Hewitt out of Wales tour|
|Kyrgios pulls out of French Open|
|Khan wants to bow out by winning blockbuster Brook fight|
|Is winter coming? Game of Thrones stars descend on Monaco|
|Root urges England´s batsmen to ´find a way´|
|Lovren, Van Dijk absolve Karius of blame|
|We all know where our nipples are - Hansen welcomes tackle trial|
|Rooney would thrive in MLS, says DC United boss|
|Pakistan complete demolition of sorry England|
|Curry, Durant reveal the secret to Thompson´s success|
|Salah will miss World Cup, claims Saudi sports administrator|
|Deschamps marvels at Zidane´s incredible Champions League success|
|FIA satisfied Ferrari ´inside the rules´|
|NSW Origin hopeful Moylan pulls the strings for rampant Sharks|
|Champions League blunders will stay with Karius - Clemence|
|Warriors assuming Iguodala won´t play game seven, Kerr says|
|Lampard confirms Derby talks over vacant managerial position|
|French Open 2018: The science behind Nadal´s clay-court dominance explained|
|Walsh to continue as Bangladesh head coach|
|Bale makes emphatic point after Champions League omission|