The main objective is to analyses the richness, abundance, and distribution of dinoflagellates in the open ocean of the southern Gulf of Mexico (hereinafter sGM). This research was conducted at 10 m depth of 63 sampling sites during the SOGOM/03 (spring/2017) and SOGOM/04 (summer/2018) oceanographic surveys aboard the R/V ”Justo Sierra”, UNAM. The analysis of dinoflagellates was performed with an inverted light microscope. Temperature, salinity, oxygen, nutrients, and chlorophyll-a data were registered. Diversity was estimated with the Shannon (H’) and Simpson (D) indexes. 128 taxa in 10 orders, 26 families, and 38 genera were recorded. In spring showed the highest richness with 124 taxa, while summer showed 83. The representative and widely distributed orders were Peridiniales, Gymnodiniales and Gonyaulacales. The most important species were Srcrippsiella trochoidea, Oxytoxum sp, Oxytoxum scolopax, Tripos teres, Gymnodinium spp, Gyrodinium spirale, Dinophysis sp. and Dinophysis rapa with cell concentrations ranging from 1.5x10 4 cell l -1 to 5.0x10 3 cell l -1. The Shannon Index in average showed higher values in summer (4.1 bits/ind), compared to spring (3.1 bits/ind), showing significant difference (p<0.05). There was evidence of a turnover in the species of the Peridiniales and the Gonyaulacales between the two seasons. The occurrence and distribution of dinoflagellates are according to their different ways of feeding, particularly heterotrophic and mixotrophic species which give them the ability to survive in the oligotrophic conditions of sGM open ocean. A list of taxa registered is provided here in. Our results represent the first report on the distribution and diversity of dinoflagellates in an extensive open ocean area of sGM that has not been studied. Thus, this information can be considered as a baseline for further studies to improve the understanding of the function of this important group of microalgae.