Chelsea is a city in Suffolk County, Massachusetts, United States, directly across the Mystic River from Boston. The 2020 census reported Chelsea as having a population of 40,787, thereby making it one of the most densely populated cities in Massachusetts only second to 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 one of the highest percentage of Latino residents in Massachusetts, only second to Lawrence.