Greencastle is a city in eastern Sullivan County, Missouri, United States.

[4] The plat for Greencastle was surveyed on March 12, 1857, at the behest of Victor Doze, Marion Overstreet, E.E.

The area had been inhabited for some time prior to 1857 however, with the first permanent home built on the present site of Greencastle by Marion Sanders circa 1853.

Moreover, the Greencastle Methodist Episcopal Church was organized in 1845 and held services just south of the town's later platted location.

The Greencastle post office was established in 1857 with I.E. Wood as first postmaster, followed a short time later by the opening of the new towns' first general store.

A creamery was established in 1885 and produced an average of 1,500 pounds of butter per week.

39.0% of all households were made up of individuals, and 24.4% had someone living alone who was 65 years of age or older.

About 19.5% of families and 27.1% of the population were below the poverty line, including 22.6% of those under the age of eighteen and 33.3% of those 65 or over.