Special routes
Secondary state highways
thumb|right|S-486 at [[Columbia Falls, Montana|Columbia Falls, August 2013]]
Montana's secondary system was established in 1942, but secondary highways (S routes) were not signed until the 1960s. and are abbreviated as "S-nnn". Route numbers 201 and higher are, with very few exceptions, exclusively reserved for S routes. Notable exceptions include, MT 287, and the former MT 789.
The highway markers for Montana's Secondary Highways are distinctive in that the route number appears in black on a white down-pointing arrowhead.