"I'm delighted to have Adrian join our stable of drivers for the 2015 season," team principal Frank Williams said of the veteran of 128 grands prix.
"Having most recently raced during the 2014 season also gives him excellent knowledge of the current generation of race car and new Hybrid power units, which is invaluable for anyone needing to step into the cockpit in 2015.
"The fight for a top Constructors' Championship position will be intense this season, therefore we have selected a driver with recent race experience and are confident that if the need arose, he would be a solid pair of hands to race the FW37," added Williams.
The former champions, who finished third overall last year, have Susie Wolff as a test driver and 21-year-old Briton Alex Lynn as a development driver but neither have raced in F1 before.
Williams's lack of a reserve ready to step in if needed at short notice was highlighted after Finland's Valtteri Bottas hurt his back in qualifying in Australia and was unable to race in the season-opener.
Bottas was passed fit at Sepang on Thursday and will return alongside Brazilian team mate Felipe Massa at the Mercedes-powered team.
Sutil lost his seat at Sauber at the end of last year and previously raced for Spyker and Force India after making his debut in 2007.
(Reporting by Alan Baldwin, editing by Sudipto Ganguly)
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