Adrian Sutil has been named reserve driver at Williams for the 2015 season.
The German is without a race seat this year after being dropped by Sauber at the end of 2014.
"I'm very proud to be a part of such a famous and great F1 team," said Sutil, who has also raced in F1 for Spyker and Force India. "To work for Sir Frank Williams and his team means a lot."
The 32-year-old will provide experienced back-up to race drivers Felipe Massa and Valtteri Bottas.
Finnish driver Bottas was forced to miss the first race of the season due to a back injury.
There had been speculation over who would replace Bottas were he unable to return for the Malaysian Grand Prix, with Williams not open to promoting test driver Susie Wolff or development driver Alex Lynn.
Bottas passed a fitness test on Thursday and will compete in Malaysia, but the addition of Sutil, who has contested 128 grands prix, offers security should a similar situation arise.
"Having most recently raced during the 2014 season also gives him excellent knowledge of the current generation of race car," said team owner Sir Frank Williams.
"If the need arose, he would be a solid pair of hands to race the FW37 and assist our 2015 campaign."
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