Species of the dinoflagellate genus Alexandrium are marine and planktonic forms, widely distributed, and some are recognized to form harmful algal blooms and to produce saxitoxins causing Paralytic Shellfish Poisoning in humans, and other toxins. We studied the species composition of Alexandrium in tropical and subtropical coastal areas of the Mexican Pacific: from the southern Gulf of California to the Gulf of Tehuantepec. Eleven Alexandrium species were identified, described, and illustrated using light microscopy, and occasionally scanning electron microscopy for certain species. Additionally, the genetic characterization of seven strains and four species, was performed, using sequences of the D1/D2 LSU rDNA and ITS regions. We identified the species (morphospecies): Alexandrium affine , A . gaarderae , A . globosum , A . leei , A . margalefii , A . minutum , A . monilatum , A. pseudogonyaulax , A . tamarense , A . tamiyavanichii , and A . tropicale . Of these, A. affine , A . leei , A . minutum , A . monilatum , A. pseudogonyaulax , A . tamarense and A . tamiyavanichii have been widely recognized as harmful algae. Alexandrium gaarderae , A . globosum and A . tropicale are new records for the Mexican Pacific. This is the first morphological documentation of A. pseudogonyaulax . Future studies of the genus might increase its species richness if more cultures are established and metabarcoding approach is used.