The 100th meridian west is a line of longitude passing through North America and the Pacific Ocean. In the Great Plains of the central United States, the meridian passes through North Dakota, South Dakota, Nebraska, Kansas, Oklahoma, and Texas. It the central Great Plains it roughly marks the western boundary of the normal reach of moist area from the Gulf of Mexico. The type of agricultural west of the meridian typically relies heavily on irrigation. Historically the meridian has often been taken as a rough boundary between the eastern and western United States.

A sign across U.S. Highway 30 in Cozad, Nebraska prominently marks the place where the meridian intersects the routes of the Emigrant Trail, Pony Express, transcontinental railroad. and the Lincoln Highway.