This is the data release for gillnet data from a 2016-18 USGS Tidal Wetlands Study and a 2010-11 Sacramento Splittail Study targeting fish in the upper San Francisco Estuary and Sacramento-San Joaquin Delta. This data was used to analyze selectivity parameters of five gillnet mesh sizes (38.1, 50.8, 63.5, 76.2, 88.9 mm) on 15 fish species of the upper San Francisco Estuary and Sacramento-San Joaquin Delta (Striped Bass (Morone saxatilis), Sacramento Splittail (Pogonichthys macrolepidotus), White Catfish (Ameiurus catus), Redear Sunfish (Lepomis microlophus), Sacramento Pikeminnow (Ptychocheilus grandis), Tule Perch (Hysterocarpus traskii), Jacksmelt (Atherinopsis californiensis), Sacramento Sucker (Catostomus occidentalis), Largemouth Bass (Micropterus salmoides), Threadfin Shad (Dorosoma petenense), Black Crappie (Pomoxis nigromaculatus), American Shad (Alosa sapidissima), Hitch (Lavinia exilicauda), Golden Shiner (Notemigonus crysoleucas), and Bluegill Sunfish (Lepomis macrochirus). Gillnet selectivity was estimated using the SELECT (share each length class’s catch total) method, which is a generalized linear model approach of indirect estimation of selectivity. Understanding gillnet selectivity can help inform future studies of fish assemblage structure in the upper San Francisco Estuary as well as studies that target fish species used in this analysis. Understanding size selectivity of the sampling gear can be helpful in determining the size structure of fish populations and decreasing bias in size-class catch data.