It travels for less than one mile (1.6 km) from Palouse Cove Road at the Washington state line to U.S. Route 95 (US-95) north of Viola.

The highway is maintained by the Idaho Transportation Department.

SH-66 begins at the Washington state line as a continuation of Palouse Cove Road, which connects to the city of Palouse.

From the state line, the highway travels southeast through a rural area and turns east to reach its terminus at US-95 north of Viola.

The route is a two-lane undivided road for its entire length.