Sutil has competed in all but one season since making his Grand Prix debut with the Spyker F1 team in 2007, but lost his seat with Sauber at the end of the 2014 campaign.
"I'm very proud to be a part of such a famous and great F1 team," he said. "To work for Sir Frank Williams and his team means a lot to me. I would like to thank everyone at Williams for their belief in my qualities and the opportunity to work with such a successful and competitive outfit."
In his new role, Sutil will prioritise spending time in the Williams simulator and on integrating himself into the team, to ensure he is ready to step into the cockpit should the need arise at any point over the 2015 campaign.
"I'm delighted to have Adrian join our stable of drivers for the 2015 season," founder and team principal Frank Williams said.
"Adrian brings extensive racing experience, having competed 128 Formula One races during his career. 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 and assist our 2015 campaign."
Sutil's appointment follows Valtteri Bottas having to sit out the 2015 season-opener in Australia on medical advice. The Finn injured his back during qualifying in Melbourne, but has been passed fit to compete in Malaysia this weekend.