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A new species of the paracalanid copepod genus BestiolinaAndronov, 1991is described from Laguna Mandinga, a coastal lagoon system of the eastern coast of the southern Gulf of Mexico. Bestiolina mexicanan. sp. most closely resembles B. sinica(Shen & Lee, 1966) and B. arabicaAli, Al-Yamani, and Prusova, 2007but can be distinguished from these and other known congeners by the number and presence of posterior surface spinules on the second and third exopodal segments of female legs 2–4, and the number of spinules on the anterior surface of the second endopodal segment of legs 2–4. This is the only species of the genus with 3, 0, 0 spines on the exopodal segments of leg 2 and 2, 3 spines on the second endopodal segments of legs 3 and 4, respectively. The mandible blade of B. mexicanan. sp. has 2 large supplementary teeth which have not been hitherto observed in the other species of the genus. This is the smallest species in the genus and has some reductions in the armature of the mouthparts. The new species has been known to occur only from this lagoonal system in the Gulf of Mexico and it is the first species of Bestiolinarecorded in the northern Atlantic Ocean. The presence of a member of this primarily Indo-Malayan genus in the northwestern Atlantic probably represents the result of its final divergence and is explained by dispersal processes related to the Pliocene conditions.