The driver, believed to be a local man in his 30s, was the only person in the vehicle travelling along Oldham Road towards Rishworth, West Yorkshire.
Police arrived at Booth Wood Reservoir near the Turnpike Public House at about 20:20 BST on Sunday.
Specialist teams are due to search the water later, West Yorkshire Police said.
The family of the driver has been informed.
Oldham Road has been closed and motorists have been advised to seek alternative routes.ã€€
Police have appealed for witnesses.
A reservoir is being searched after a car left the road and went into the water.