Texas State Highway 166

State Highway 166 (SH 166) runs for 43.0 miles (69.2 km) from SH 118 near Nunn Hill in northern Jeff Davis County, Texas passing around a major portion of the Davis Mountains before ending at SH 17 about two miles south of Fort Davis. The highway is maintained by the Texas Department of Transportation (TxDOT).

The scenic highway was originally a longer loop around the mountain range and was envisioned in 1927 as a "state park highway" until an actual state park could be established in the area. SH 166 facilitated construction of Davis Mountains State Park and the University of Texas at Austin's McDonald Observatory during the 1930s.

SH 166 is located entirely within Jeff Davis County and forms part of the route linking Fort Davis, the county seat, to Valentine, the county's only incorporated town. The highway link to Valentine is the only highway intersection between the termini of SH 166, and the rural route does not pass through any cities or towns.