The 30-year-old Scot raced for Force India in 58 grands prix between 2011 and 2013, scoring 121 points, and became Williams' reserve last year.
He will attend races in his role, ready to stand in if needed.
"His extensive Formula One knowledge will once again be invaluable, especially with new regulations in place for the 2017 season," said deputy team principal Claire Williams in a statement.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Mercedes-powered Williams have 18-year-old Canadian rookie Lance Stroll as one of their race drivers, along with Brazilian veteran Felipe Massa.
Rule changes, with wider tyres and faster cornering speeds, are expected to make the cars harder to handle and more of a physical challenge for drivers.
(Reporting by Alan Baldwin, editing by Amlan Chakraborty)
