Cumberland County is a county in the U.S. state of Maine. As of the 2020 census, the population was 303,069, making it the most populous county in Maine. Its county seat is Portland. Cumberland County was founded in 1760 from a portion of York County, Province of Massachusetts Bay, and named for William, Duke of Cumberland, a son of King George II. Cumberland County has the deepest and second-largest body of water in the state, Sebago Lake, which supplies tap water to most of the county. The county is the state's economic and industrial center, having the resources of the Port of Portland, the Maine Mall, and having corporate headquarters of major companies such as Fairchild Semiconductor, IDEXX Laboratories, Unum, and TD Bank. Cumberland County is part of the Portland–South Portland, ME Metropolitan Statistical Area.

Portland is the most populous city in the U.S. state of Maine and the seat of Cumberland County. Portland's population was 68,408 in April 2020. The Greater Portland metropolitan area has a population of approximately 550,000 people. Historically tied to commercial shipping, the marine economy, and light industry, Portland's economy in the 21st century relies mostly on the service sector. The Port of Portland is the second-largest tonnage seaport in the New England area as of 2019.