Route 21, also known as Bunbury Road and Fort Augustus Road, is a 30.9 km (19.2 mi), two-lane, uncontrolled-access, secondary highway in central Prince Edward Island.

The route continues east as Bunbury Road and crosses Fullerton Creek, leading to a roundabout with Route 215.

The route proceeds north from the roundabout as Fort Augustus Road before turning to the east again at Mermaid.

The route then crosses several more rivers, including the Pisquid River, and intersects Route 22 (Mount Stewart Road) at Pisquid.

The route continues eastward as an unpaved rural road, ending at Route 323 (Saint Patrick's Road).