Waldo County is a county in the state of Maine, United States. As of the 2020 census, the population was 39,607. Its county seat is Belfast. The county was founded on February 7, 1827, from a portion of Hancock County and named after Brigadier-General Samuel Waldo, proprietor of the Waldo Patent.

Belfast is a city in Waldo County, Maine, United States. As of the 2020 census, the city population was 6,938. Located at the mouth of the Passagassawakeag River estuary on Belfast Bay and Penobscot Bay. Belfast is the county seat of Waldo County. Its seaport has a wealth of historic architecture in several historic districts, and remains popular with tourists.