The presence of salinity in shallow waters influences living resources and habitats within Gulf of Mexico estuaries. The salinity gradient is widely recognized as foundational in maintaining biological diversity and productivity of estuaries. A clear understanding of the factors controlling salinity and its variability in estuarine surface waters is essential for proper stewardship and for sustaining ecological structure and function. Salinity data are collected by numerous Federal, State, and local agencies and universities as part of routine data-collection programs. The U.S. Geological Survey compiled salinity data from existing online databases – all water samples were collected in Gulf of Mexico estuaries. The primary criterion for data from a station to be included in the compilation was a lengthy record of continuous collection using a data sonde programmed to at least hourly intervals. Stations that represented full estuarine gradients, from fresh to saline, were prioritized. Data were compiled from salinity stations in the five Gulf states and combined into one .txt file and one .feather file. Continuous data collection of salinity concentrations began at a few stations in the mid-1980s, and the number of stations with data sondes has increased over time for a total of 532,076 observations at 92 stations provided in this data release.