The Pole returned to Formula 1 after eight years away from the grid recovering from life-altering injuries sustained in a rally crash.
But things have not gone to plan for driver or team with Williams often trailing at the back of the field.
Kubica has scored the team's sole point of the season so far in Germany, but will depart for new opportunities in 2020.
He said: "We are looking forward to next year and looking for different opportunities. I've taken the decision I will not continue with Williams. I will stop at the end of the year.
?️ #RK88: "I would like to thank the team for the last two years and for helping make my comeback to the @F1 grid possible. I have enjoyed my time with ROKiT Williams Racing...but I feel that the time is now right for me to move on to the next chapter in my career.” pic.twitter.com/D7KdDovSFw
"It's a decision I took. I will see what opportunities are there for the future."
Kubica has recently been linked with a switch to DTM, but is also reported to have held talks with Racing Point ahead of a potential development role, having done similar for Williams before stepping up to the race line-up.
He added: "I always said that I took a lot of energy, a lot of time for me to come back to the sport.
"Since I joined Formula 1 [again] I said I would like to stay. I said this year would be a goal to remain in Formula 1.
"My answer is yes [I want to stay in F1] but not for every cost.
"I have to do what will bring me back a bit of joy in racing. This season has been very tough. Being back in Formula 1 after many years away is not easy, especially when we're in this situation, but I have to thank the team."
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