A number of harpacticoid copepods were collected during two series of sampling campaigns in a coastal lagoon and in a brackish system in northern and southern Sinaloa (northwestern Mexico), respectively. Four enhydrosomid species were recognized. Three of them are new to science. Enhydrosoma lacunae is reported for the sixth time. This record extends the distribution of the species from the Caribbean Sea to the Eastern Tropical Pacific, and it is suggested that it is an invasive species borne in water ballast of ships. Enhydrosoma parapropinquum sp. nov. bears a strong resemblance to E. propinquum, but these species are different in shape of the caudal rami and female fifth leg. Enhydrosoma casoae sp. nov. and E. solitarium sp. nov. showed to be closely related to E. vicinum. These three species seem to be related by the structure and armature formula of the female fifth swimming leg. Enhydrosoma casoae sp. nov. and Enhydrosoma solitarium sp. nov. can be separated from each other by the ventral spinular ornamentation of urosome and spinular ornamentation of female P5 baseoendopod.