It runs for 7.83 miles (12.60 km), connecting the towns of South Berwick and York.
SR 91 begins at an intersection with U.S. Route 1 (US 1) in York.
The highway proceeds out of town to the northwest towards South Berwick, just nicking the corner of Eliot along the way.
The first three miles (4.8 km) of the route follow the York River towards South Berwick.
The entire route is in York County.