Nashua is a city in southern New Hampshire, United States. As of the 2020 census, it had a population of 91,322, the second-largest in nort…Nashua is a city in southern New Hampshire, United States. As of the 2020 census, it had a population of 91,322, the second-largest in northern New England after nearby Manchester. It is one of two county seats of New Hampshire's most populous county, Hillsborough; the other being Manchester.