Spatial variations of polychaetes along the Baja California Peninsula continental shelf were analysed in two ways: (1) by evaluating the species richness and defining the faunal assemblages from local sampling, and (2) by characterizing the latitudinal variation of beta-diversity based on the distribution of all polychaetes reported up to now. Twenty-seven stations from three oceanographic expeditions were sampled, and 2858 individuals from 38 families and 231 species were identified. Polychaetes were abundant (mean = 37.5 ind 0.1 m−2) and diversified (mean = 15 species station−1). Differences in species composition defined seven faunal assemblages, but their number of species did not show significant latitudinal changes. The beta-diversity analysis was based on distribution data of 730 species from 47 families. The presence of a group of islands in the middle Gulf could have a negative effect on the species distribution, since the lowest values of β
T-diversity (0.39) were found in the northern Gulf, but their species were different from those recorded in the central region. The wide variation in β
T-diversity (0.5–0.87) showed latitudinal changes in the species composition, mainly in the Gulf mouth (0.86–0.87), which indicated that the fauna inside the Gulf was different from that inhabiting the Pacific coasts. The ICE and Chao2 estimators showed that the polychaetes in the study area are relatively well known (>80%), and that most species (415) have small distribution ranges. The effects of these infrequent species were similar at each latitudinal band, which suggested that the observed β
T-diversity pattern could represent a suitable estimation.