San Antonio Bay is a bay on the Texas Gulf Coast situated between Matagorda and Aransas Bay. It consists mainly of the combined waters of the San Antonio and Guadalupe rivers, and is located at the mouth of the Guadalupe River, about 55 miles (89 km) northeast of Corpus Christi and 130 miles (209 km) southeast of San Antonio. It is protected from the Gulf of Mexico by Matagorda Island, leaving only relatively small and distant outlets to the Gulf for little mixing of bay and Gulf waters. The remoteness of the bay has prevented the establishment of major ports as seen on Aransas Bay and Corpus Christi Bay, to the south.

Aransas Bay is a bay on the Texas Gulf Coast, approximately 30 miles (48 km) northeast of Corpus Christi, and 173 miles (278 km) south of San Antonio. It is separated from the Gulf of Mexico by San José Island. Aransas Pass is the most direct navigable outlet into the Gulf of Mexico from the bay. The cities of Aransas Pass and Port Aransas are located at the southern end, and Rockport is found on the central western shore. The bay is oriented laterally northeast–southwest, and is extended by Redfish Bay to the southwest, Copano Bay to the west, Saint Charles Bay to the north, and Mesquite Bay to the northeast. Aransas Bay is part of the Mission-Aransas National Estuarine Research Reserve.