The signed northern terminus of SR 423 is at its junction with SR 231 nearly 3 miles (4.8 km) east of Morral.

However, SR 423 is defined to continue northeast from this point along SR 231 for about 0.25 miles (0.40 km) before coming to an end at the U.S. Route 23 (US 23) expressway.

[5][6] The route number was originally designated for a planned bypass around Portsmouth in the 1960s (now designated as Ohio State Route 823).

[7][8] Between 1967 and 1969, US 23 was moved onto a new four-lane highway and the old route of US 23 became SR 423.

[9][10] The entire route is in Marion County.