Here, we provide the first list of shallow-water hydroids for Alacranes Reef, Mexico, comprising 49 species in 33 genera. The most speciose superorder was Leptothecata (36 species), with three particularly diverse families: Clytiidae, Sertulariidae, and Plumulariidae. Three species were recorded for the first time in the Gulf of Mexico (GoM), and 13 for the south-central GoM. The majority of fauna was substrate generalist with specific distributions: 38 hydroid species occurred on natural reefs, 18 on artificial reefs, 9 on drifting Sargassum spp., and 8 on the seagrass Thalassia testudinum K.D. Koenig. Our results significantly increase knowledge of the hydroids from Mexican waters. This is an important step toward detecting non-native species in the near future, as well as for improving and designing new conservation programs in the area.