Galveston County is a county in the U.S. state of Texas, located along the Gulf Coast adjacent to Galveston Bay. As of the 2020 census, its population was 350,682. The county was founded in 1838. The county seat is the City of Galveston, founded the following year, and located on Galveston Island. The most-populous municipality in the county is League City, a suburb of Houston at the northern end of the county, which surpassed Galveston in population during the early 2000s.

Galveston Bay is a bay in the western Gulf of Mexico along the upper coast of Texas. It is the seventh-largest estuary in the United States, and the largest of seven major estuaries along the Texas Gulf Coast. It is connected to the Gulf of Mexico and is surrounded by sub-tropical marshes and prairies on the mainland. The water in the bay is a complex mixture of sea water and fresh water, which supports a wide variety of marine life. With a maximum depth of about 10 feet (3 m) and an average depth of only 6 feet (2 m), it is unusually shallow for its size.