Cumby is a city in Hopkins County, Texas, United States. The population was 777 at the 2010 census, up from 616 at the 2000 census.
Originally known as Black Jack Grove as early as 1848, from 1857 to 1858 the post office was renamed to Theodocias. It was renamed again to Black Jack Grove before being renamed a final time to Cumby in 1896. It was named for Confederate army veteran Robert H. Cumby (1825-1881).
Cumby is located in western Hopkins County at 33°8′7″N 95°50′25″W (33.135235, -95.840141). Interstate 30 runs through the south side of the city, with access from Exit 110. I-30 leads east 14 miles (23 km) to Sulphur Springs, the Hopkins county seat, and west 65 miles (105 km) to downtown Dallas.
According to the United States Census Bureau, the city has a total area of 1.0 square mile (2.6 km2), of which 0.004 square miles (0.01 km2), or 0.50%, are water. The city sits on a watershed divide, with the north side draining towards the South Sulphur River, part of the Red River watershed, and the south side of the city draining towards Lake Fork Creek, part of the Sabine River watershed.