Millsboro is a town in Sussex County, Delaware, United States. The population was 3,877 at the 2010 census, an increase of 64.3% over the past decade. It is part of the Seaford, Delaware Micropolitan Statistical Area.
On July 21, 1930, the temperature in Millsboro rose to 110°F (43°C), the highest temperature ever recorded in Delaware. However, on January 17, 1893, the temperature fell to -17°F (-27°C), the coldest temperature ever seen in Delaware. Thus, it is only one of two cities in the United States to record both its state's extreme temperatures. The other city is Warsaw, Missouri.