Introduction: Sponges are increasingly dominant components of coral reefs in the Tropical Western Atlantic, yet patterns of their diversity across spatial scales remain poorly quantified. Methods: We applied a hierarchical sampling design across 13 reefs and five spatial scales —ecoregions (thousands of kilometers), subregions (hundreds of kilometers), coral reefs (tens of kilometers), localities (hundreds of meters), and quadrats (meters)— to assess the relative influence of ecological processes structuring sponge assemblages. Surveys were conducted across 13 coral reefs in the Mexican Caribbean (MC) and the Southern Gulf of Mexico (SGM), including the neritic zones of Campeche/Yucata´n (NCY) and Veracruz Neritic zone (VN). In total, 624 quadrats were examined across 60 localities within the 13 reefs. Results: Most of the explained variation in assemblage structure occurred at the local (22%) and reef (19%) scales, while regional effects were weak. Gamma diversity declined along a geographic gradient from the MC to the VN, with observed richness ranging from 95 to 63 species and extrapolated estimates reaching 122 and 73, respectively. Sample completeness profiles showed high coverage across subregions (= 0.94 for q = 1 and 2), confirming the robustness of interregional comparisons. Beta diversity among subregions was high and consistently dominated by species turnover (BJTU range: 0.64–0.73), indicating that environmental filtering, rather than nested species loss or limited connectivity, drives compositional differences. Discussion: Assemblages in the MC exhibited higher richness but lower sponge abundance, likely shaped by oligotrophic conditions and less-impacted predator communities. In contrast, NCY reefs showed moderate richness and high speciesturnover, reflecting strong environmental heterogeneity, while VN reefs exhibited low richness but high abundance, likely driven by high sedimentation and reduced top-down control. Our findings underscore the central role of small-scale processes in shaping sponge diversity and caution against assuming regional nestedness in reef communities