Chelsea is a city in Suffolk County, Massachusetts, United States, directly across the Mystic River from Boston. At the 2020 census, Chelsea had a population of 40,787, making it the second most densely populated city in Massachusetts, behind Somerville. With a total area of 2.46 square miles (6.4 km2), Chelsea is the smallest city in Massachusetts in terms of total area. It is the city with the second-highest percentage of Latino residents in Massachusetts, behind Lawrence.

Suffolk County is located in the Commonwealth of Massachusetts, in the United States. As of the 2020 census, the population was 797,936, making it the fourth-most populous county in Massachusetts. The county comprises the cities of Boston, Chelsea, Revere, and Winthrop. The traditional county seat is Boston, the state capital and the largest city in Massachusetts. The county government was abolished in 1999, and so Suffolk County today functions only as an administrative subdivision of state government and a set of communities grouped together for some statistical purposes. Suffolk County is located at the core of the Boston-Cambridge-Newton, MA-NH Metropolitan Statistical Area as well as the greater Boston-Worcester-Providence, MA-RI-NH-CT Combined Statistical Area.