Marshallberg is an unincorporated area and census-designated place in Carteret County, North Carolina, United States. As of the 2010 census, it had a population of 403. It lies on the mainland, directly across Core Sound from Harkers Island and Cape Lookout beyond that. Lying at the extreme southeast corner of a peninsula, it is isolated from other Carteret County communities and only a single road connects it to the rest of the state.