The Laguna Madre of Mexico is the second biggest hypersaline coastal lagoon from the Gulf of Mexico, and supports the most important artisanal shrimp fishery in the country. The population size structure of penaeids from this artisanal fishery was examined, analyzing temporal and interspecific variations. The results indicate that the shrimp fishery in Laguna Madre is supported by three penaeid species (Farfantepenaeus aztecus, F. duorarum, and Litopenaeus setiferus); the consistent presence of F. aztecus and F. duorarum caught throughout the year contrasted with the scarce occurrence of L. setiferus, which exhibited significant fluctuations in its relative contribution to shrimp catches. Significant differences in monthly mean size were observed for the three species. Larger individuals were caught in colder months (November–March); however, the body condition of shrimp was higher in warmer months (April–October). A negative relationship between shrimp size and water temperature was found, which could suggest differences in size at emigration according to temperature fluctuations.