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The aim of this study was to analyze the seasonal and spatial variations of the Spionida, one of the most abundant and diverse polychaete groups in the Gulf of California. A total of 6527 organisms from 39 stations taken during the winter‐spring and summer periods were identified. The number of species was similar in winter‐spring and summer (29–30), but abundance was significantly higher during the winter‐spring season (4791 versus 1736 in summer). The overall highest density values were recorded in the central‐eastern region (33.45–164.66 org./0.1 m2) where some of the areas with the highest species richness were found (12–16 species). A similar pattern was observed during summer, but with lower values of density. Paraprionospio pinnata, the key species, contributed with 16.24% to the total average dissimilarity between seasons and defined most assemblages of species. It was mainly distributed at stations deeper than 64 m in the eastern shores. The density and composition of species change spatially as a response to changes in depth and sand percent; seasonally, the temperature influenced the variation in the faunal distribution. [ABSTRACT FROM AUTHOR]