The genus Meristotheca J. Agardh is the second largest in Solieriaceae with 13 species widely distributed whose thalli are mainly flattened. On the basis of molecular and morpho-anatomical evidence, M. spinella Núñez-Resendiz, Dreckmann & Sentíes, sp. nov. is the second species in the genus with cylindrical axes to be described from the southwestern Gulf of Mexico. rbcL sequences and genetic distance values indicate its separation from other species within Meristotheca and from other morphologically similar species occurring in its known distribution range as M. cylindrica Núñez-Resendiz, Dreckmann & Sentíes. The numerous short branchlets that cover the margins of main axes and branches, among other relevant anatomical and reproductive characters, is the main difference from M. cylindrica, its sister species.