Union is a town located in the northeastern part of Tolland County, Connecticut, United States and is part of the Northeastern Connecticut Planning Region and Last Green Valley National Heritage Corridor. The population was 785 at the 2020 census, making it the least populous town and municipality in Connecticut. Union includes the village of Mashapaug located in the Eastern Uplands at the Massachusetts state line. Union is located 33 miles northeast of the state capital, Hartford and 67 mi (108 km) southwest of Boston as well as 153 mi (246 km) northeast of New York City.

The town is the basic unit of local government and local division of state authority in the six New England states. Most other U.S. states lack a direct counterpart to the New England town. New England towns overlay the entire area of a state, similar to civil townships in other states where they exist, but they are fully functioning municipal corporations, possessing powers similar to cities and counties in other states. New Jersey's system of equally powerful townships, boroughs, towns, and cities is the system which is most similar to that of New England. New England towns are often governed by a town meeting, an assembly of eligible town residents. The great majority of municipal corporations in New England are based on the town model; there, statutory forms based on the concept of a compact populated place are uncommon, though elsewhere in the U.S. they are prevalent. County government in New England states is typically weak at best, and in some states nonexistent. Connecticut, for example, has no county governments, nor does Rhode Island. Both of those states retain counties only as geographic subdivisions with no governmental authority, while Massachusetts has abolished eight of fourteen county governments so far. Counties serve mostly as dividing lines for the states' judicial systems and some other state services in the southern New England states, while providing varying services in the more sparsely populated three northern New England states.