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.

An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.