S gauge is a scale in model railroading. Modeled at 1:64, S gauge runs on two-rail track with the rails 7/8 inch (22.2 mm) apart. S gauge is sometimes confused with standard gauge, a large-scale standard for toy trains in the early part of the 20th century.

1:64 scale trains first appeared in England in the early 20th century. The first trains marketed as S gauge appeared in the 1930s when American Flyer marketed a 1:64 scale train that ran on 3-rail track similar to that of Lionel. Following World War II, American Flyer introduced an S-gauge train running on two-rail track for greater realism, and S gauge entered its heyday, which ended around 1959. However, during that period, Lionel outsold American Flyer nearly 2 to 1. American Flyer's parent company went out of business and the brand was sold to Lionel in 1967.

Lionel reintroduced S gauge trains and accessories under the American Flyer name in 1979. Another manufacturer, American Models, entered the marketplace in 1981. Because O gauge's scale ranges between 1:43 and 1:64 scale, many manufacturers of buildings and other accessories pick an in-between scale so they can market both to O- and S-gauge hobbyists.