Pennsylvania Route 166 is a north–south state highway in Fayette County, Pennsylvania. It follows the path of the Monongahela River along the entire western border of Fayette County.

Fayette County is a county in the Commonwealth of Pennsylvania. It is located in southwestern Pennsylvania, adjacent to Maryland and West Virginia. As of the 2020 census, the population was 128,804. Its county seat is Uniontown. The county was created on September 26, 1783, from part of Westmoreland County and named after the Marquis de Lafayette. The county is part of the Southwest Pennsylvania region of the state.