A supplemental route is a state secondary road in the U.S. state of Missouri, designated with letters. Supplemental routes were various roads within the state which the Missouri Department of Transportation was given in 1952 to maintain in addition to the regular routes, though lettered routes had been in use from at least 1932. The goal of the secondary highway system was to place state-maintained roads within 2 miles (3 km) of more than 95% of all farm houses, schools, churches, cemeteries and stores. The four types of roads designated as Routes are:
Supplemental routes make up 19,064 miles (30,681 km) (59%) of the state highway system.
The more major supplemental routes of the system are ones assigned with single-letter designations (such as "K"). Minor branch routes and farm-to-market roads, which often end at county roads or are former alignments of the other highways, are typically assigned with two-letter designations consisting of two of the same letter (e.g. "KK"). Additionally, combinations of letters may be used, but always with A as the first letter (such as "AD"); the only exception of this is on Route BA in western St. Louis County. Combinations beginning with the letter R are also used for routes that connect with state parks or other recreational facilities, which is the only use of R on the system; Route AR, a non-recreational route south of Bakersfield, is the only exception to this.