<p>Sediments are a main component of the coral reef framework, which are composed by calcareous bioclasts, non-skeletal carbonates, and terrigenous siliciclasts. Extensive work has been done to determine the bioclast composition of the coral reefs in the Western Tropical Atlantic. Notwithstanding, the sediment composition of coral reefs in the Southwestern Gulf of Mexico, which are influenced by fluvial discharge, remains unknown. Thus, the aim of this study was to investigate the bioclast composition of the Southwestern Gulf of Mexico reefs, and their differences to the reefs of the Mexican Caribbean, and those of the Western Tropical Atlantic. Surface sediment samples were collected at the reef flat zone of ten coral reefs in the Southwestern Gulf of Mexico and the Mexican Caribbean. From the results it can be concluded that the clast composition of reef sediments is the result of several processes with high spatio-temporal variability. Terrigenous siliciclasts are an important component of reef sediments in the Southwestern Gulf of Mexico. The clast composition of the Southwestern Gulf of Mexico (Scleractinia &gt; Aggregates &gt; <i>Halimeda</i> &gt; CCA &gt; Gastropoda), is different to that of the Mexican Caribbean and the Western Tropical Atlantic reefs. Applying statistical multivariate analyses, we found that four groups of reefs differ in their clast composition, the Eastern Gulf of Mexico and Florida dominated by <i>Halimeda</i>, the Bahamas with non-skeletal clasts as the most abundant, and the fourth group formed by the fringing reefs of Roatan, and some other localities, with Scleractinians as the dominant clast. Also, clast composition differs between oceanic reefs (<i>Halimeda</i>-dominated), nearshore reefs (Scleractinia-dominated), and carbonate platforms (abiotic clasts dominated). In average the most abundant bioclasts in the Western Tropical Atlantic reefs are Scleractinians &gt; <i>Halimeda</i> &gt; Mollusks &gt; non-skeletal clasts &gt; calcareous macroalgae.</p>