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.