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Background: There are few studies about Nemacystus on the Atlantic coast, and N. howei has been reported only once for Mexico; that record, however, did not provide a description of this species. Question: What are the morphological characteristics of Nemacystus howei and its distribution on the Yucatan coast? Species study: Macroalgae, Phaeophyceae, Nemacystus howei. Study site and year of study: North coast of Yucatán Peninsula of Mexico. Dry and rainy seasons from 2017 to 2019. Methods: Sampling was carried out at five localities, at 5 and 10 m depths, with scuba equipment. Specimens were preserved in diluted formalin in sea water; silica gel and dried-pressed vouchers are housed at the UADY Results: Six samples were collected at the five localities. Each one contains a small mass of filamentous of specimens of Nemacystus howei , that were found only during dry season, at both depths sampled. This alga has a thallus of an entangled filamentous mass of cylindrical, mucilaginous axes, of a light brown color. Younger branches have a solid central axis. Branching is abundant. The main, older axes are hollow. The assimilatory filaments are branched a few times, producing uniseriate colorless hairs and with plurilocular sporangia arising from the base. Nemacystus howei is easily confused with Cladosiphon occidentalis Kylin due to their similar morphology, branching pattern, and size. Conclusion: The significant morphological similarities between these species as well as the paucity of specific works on the Phaeophyceae of the Yucatan Peninsula may explain why Nemacystus howei had not been previously reported.