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. With a total area of just 2.46 square miles, Chelsea is the smallest city in Massachusetts in terms of total area. It is the second most densely populated city in Massachusetts, behind Somerville, and is the city with the second-highest percentage of Latino residents in Massachusetts, behind Lawrence.