State Route 671 (SR 671) is an east-west state highway in the southern portion of Ohio. Existing entirely within Vinton County, the western terminus of this short connector route is at State Route 327 3 miles (4.8 km) northeast of the hamlet of Londonderry. Its eastern terminus is at U.S. Route 50 in the unincorporated community of Ratcliffburg.

Starting from a T-intersection with State Route 327 in Eagle Township just inside of the Ross-Vinton county line, where State Route 327 forms the western and northern legs of the intersection, State Route 671 heads easterly across the Salt Creek and through grassland through the Dixon Mill Road intersection, where it then turns to the southeast into primarily forested terrain. The highway passes back-to-back T-intersections with two unpaved roads, Fout Road and State Ford Road, while continuing to trend southeasterly through the woods to the point where it comes to an end in the Harrison Township hamlet of Ratcliffburg at a T-intersection with U.S. Route 50.

State Route 671 was originally designated in 1939 along the routing that it currently occupies between State Route 327 and U.S. Route 50. No major changes have taken place to State Route 671 since its debut.

The entire route is in Vinton County.