Williams has confirmed that former Force India and Sauber pilot Adrian Sutil will act as the squad's reserve driver this season.
Sutil made his Formula 1 debut with Spyker in 2007 and remained with the Silverstone-based squad until he switched to Sauber for 2014, where he was unable to score a point.
Sutil joins test driver Susie Wolff and development driver Alex Lynn as back-up to Felipe Massa and Valtteri Bottas.
"I’m very proud to be a part of such a famous and great Formula 1 team," said Sutil, who will spend time in the squad's simulator.
"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."
Williams added that he was pleased to sign a driver with experience of the V6 power units.
"I’m delighted to have Adrian join our stable of drivers for the 2015 season," he said. "Adrian brings extensive racing experience, having competed 128 Formula 1 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."