State Route 66 is a state-maintained highway in eastern Tennessee, including a four-lane divided highway segment in Sevier County and a two-lane segment through mountainous terrain to the northeast.

The four-lane portion of State Route 66 extends from Interstate 40 in the north and ends at U.S. Highway 441 and U.S. Highway 411 in Sevierville It is a popular route for tourists traveling to the Great Smoky Mountains National Park. This route is part of the Great Smoky Mountains Parkway.

The secondary Tennessee State Route 66 extends from U.S. Highway 25W/70 in Dandridge to Tennessee State Route 31 just south of Sneedville, crossing U.S. Highway 11E in Morristown. Between Dandridge and Morristown it is a two-laned curvy road over moderately mountainous terrain. It then follows U.S. Highway 11E to Bulls Gap, Tennessee, where it turns to the north as a Primary highway going into Rogersville, Tennessee. Then it veers northwesterly, as a secondary highway again, until its terminus at Tennessee State Route 31 just south of Sneedville, Tennessee. This section of Tennessee State Route 66 is around 60 miles (97 km).

Theoretically, the two segments are linked by US Highway 25W/70 and Interstate 40, but the route is not signed on this segment.

There are currently plans to expand TN 66 from Interstate 40 to US 441/411. This has been divided into 3 separate sections, each of which will be expanded at different times. When completed, this route will be a six-lane divided highway. The section closest to Sevierville was expanded first. This section stretched from Boyds Creek Highway to Nichols Street and was completed November 2011. The second section from Interstate 40 to Douglas Dam Road (TN 139) is now under construction with a scheduled completion date of November 30, 2012. The middle section from Douglas Dam Road to Boyds Creek Highway will be expanded last when funding is available.