Dorchester is an unincorporated community in northwestern Allamakee County, Iowa, United States.

[3] Although Dorchester is unincorporated, it has a post office, with the ZIP code of 52140,[4] which opened on 21 May 1857.

It was worse hit on 8 June 2008 and succeeding days: over seven inches (over 18 cm) of rain fell on the eighth, and resulting record high floods of 22½ feet (20½ m) almost destroyed a trailer park that was a significant part of the tiny community.

[11][12] An LP gas tank from Dorchester was discovered just eight hours later in Lansing, on the opposite side of the county.

[13] All students in the area are enrolled in the Allamakee Community School District in Waukon, even though Dorchester is closer to Spring Grove, Minnesota.