Long Island Sound is an estuary

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

 of the Atlantic Ocean

Atlantic Ocean

The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

, located in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 between Connecticut

Connecticut

Connecticut is a state in the New England region of the northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, and the state of New York to the west and the south .Connecticut is named for the Connecticut River, the major U.S. river that approximately...

 to the north and Long Island

Long Island

Long Island is an island located in the southeast part of the U.S. state of New York, just east of Manhattan. Stretching northeast into the Atlantic Ocean, Long Island contains four counties, two of which are boroughs of New York City , and two of which are mainly suburban...

, New York

New York

New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...

 to the south. The mouth of the Connecticut River

Connecticut River

The Connecticut River is the largest and longest river in New England, and also an American Heritage River. It flows roughly south, starting from the Fourth Connecticut Lake in New Hampshire. After flowing through the remaining Connecticut Lakes and Lake Francis, it defines the border between the...

 at Old Saybrook, Connecticut

Old Saybrook, Connecticut

Old Saybrook is a town in Middlesex County, Connecticut, United States. The population was 10,367 at the 2000 census. It contains the incorporated borough of Fenwick, as well as the census-designated places of Old Saybrook Center and Saybrook Manor.-History:...

, empties into the sound. On its western end the sound is bounded by the Bronx and Westchester County, New York

Westchester County, New York

Westchester County is a county located in the U.S. state of New York. Westchester covers an area of and has a population of 949,113 according to the 2010 Census, residing in 45 municipalities...

, and connects to the East River

East River

The East River is a tidal strait in New York City. It connects Upper New York Bay on its south end to Long Island Sound on its north end. It separates Long Island from the island of Manhattan and the Bronx on the North American mainland...

. On its eastern end it opens to Block Island Sound

Block Island Sound

Block Island Sound is a strait in the open Atlantic Ocean, approximately wide, separating Block Island from the coast of Rhode Island in the United States...

.
Several major cities are situated along Long Island Sound and more than 8 million people live within its watershed.