Castlewood is a city in Hamlin County, South Dakota, United States.

It is part of the Watertown, South Dakota Micropolitan Statistical Area.

[6] The city was named Castlewood after a place mentioned in the 1857 novel The Virginians by William Makepeace Thackeray.

[7] A destructive EF2 tornado hit the town on May 12, 2022, destroying several structures, damaging homes, and injuring one person.

[9] According to the United States Census Bureau, the city has a total area of 1.15 square miles (2.98 km2), all land.

The winters are long and harsh, with an annual average extreme minimum temperature of -28.1 °F (-33.4 °C), placing it within the 4a USDA Hardiness Zone (-30°F to -25°F) or (-34.4°C to -31.7°C).

[10] Summers are warm, with an average temperature of 69.4 °F (20.8 °C) in July, and experience a high level of precipitation compared to the dry winters.

38.1% were of German, 18.4% Norwegian, 9.9% Dutch, 6.8% American, 6.4% Irish and 5.6% English ancestry.