The one-time Grand Prix winner announced his departure in today's press conference in Singapore ahead of this weekend's Grand Prix at the Marina Bay Circuit.
Many drivers could be lined up to replace the 34-year-old, with Formula 2 driver Nicholas Latifi currently the favourite to take the seat in 2020. However, Nico Hülkenberg could also be an option, with the German having lost his drive at Renault for 2020 at the hands of Esteban Ocon.
In a statement released by Williams, Kubica thanked the team for the trust they put in him to allow him to make a comeback in the first place, having not driven in F1 since 2010 before this year's Australian Grand Prix.
“I would like to thank the team for the last two years and for helping make my comeback to the Formula One grid possible," he said.
"I have enjoyed my time with ROKiT Williams Racing, both as Reserve & Development Driver and as a Race Driver this year, but I feel that the time is now right for me to move on to the next chapter in my career.”
Despite scoring Williams' only point this season, Kubica has been beaten by teammate George Russell on all occasions so far this season, with the margin sometimes being over a second between the two.
Deputy team principal Claire Williams also came out to express her gratitude for Kubica's services.
“I would like to thank Robert for his hard work and respect his decision to leave the team at the end of the 2019 season.
"Robert has been an important member of the team in both his role as Reserve & Development Driver, and subsequently as one of our Race Drivers in 2019. We thank him for his continued efforts throughout what has been a challenging couple of seasons and wish him well in his future endeavours.”
ROKiT Williams Racing can confirm that Robert Kubica has decided to leave the team at the end of the 2019 Formula One racing season.— ROKiT WILLIAMS RACING (@WilliamsRacing) September 19, 2019