The seasonal circulation on the western shelf of the Gulf of Mexico is studied using a high-resolution numerical simulation, historical hydrographic data, sea level data, and satellite images. Three regions are distinguished, the Tamaulipas-Veracruz (TAVE) shelf, the Louisiana-Texas (LATEX) shelf, and the western Campeche Bank. On the TAVE shelf there is a swift reversal of the along-shelf current, downcoast from September to March and upcoast from May to August when there is upwelling due to offshore Ekman transport. Circulation on the western Campeche Bank is upcoast throughout the year. The LATEX shelf has a cyclonic circulation, except during summer months when the flow is eastward. During spring-summer the upcoast current on the TAVE shelf reaches the southern Texas shelf where it encounters a downcoast coastal current favoring offshore transports. In the fall-winter, the downcoast current reaches the southern Bay of Campeche where it meets an opposing along-shelf current, generating seasonal offshore transports. During fall and winter, cool low-salinity water from the Mississippi and Atchafalaya Rivers is advected westward along the LATEX shelf onto the TAVE shelf, developing along-shelf fronts and temperature inversions commonly observed over the outer shelf and shelf break. The main forcing over the western shelf of the gulf is the along-coast wind stress component. The existence of the cross-shelf transports in the confluence regions is supported by chlorophyll a data. Up to 80% of the seasonal sea level variability is explained by the along shelf currents and the low-frequency variability of the atmospheric sea level pressure.