Brockton is a city in Plymouth County, Massachusetts, United States; the population was 105,643 at the 2020 United States census. Along with Plymouth, it is one of the two county seats of Plymouth County. It is the sixth-largest city in Massachusetts and is sometimes referred to as the "City of Champions", due to the success of native boxers Rocky Marciano and Marvin Hagler, as well as its successful Brockton High School sports programs.