Geodiversity is increasingly recognized as a fundamental component of nature and an essential substrate for biodiversity. Despite methodological advances, quantitative tools for assessing abiotic heritage in extensive, low-relief coastal plains remain scarce for informing territorial management. This study presents the first integrated geodiversity assessment of the Coastal Plain of Rio Grande do Sul (CPRS), Brazil (~33,000 km2), using a Geodiversity Index (GI) based on eight variables across four sub-indices (geolodiversity, geomorphodiversity, pedodiversity, hydrodiversity) quantified in 292 grid cells (189 km2 each). Results reveal that 26% of the CPRS exhibits High or Very High geodiversity, concentrated in two distinct hotspots: a northern sector controlled by geological-geomorphological complexity, and a southern sector dominated by hydrodiversity of the Patos-Mirim estuarine system. Strong spatial convergence with biodiversity priority areas reinforces geodiversity as nature's stage and supports integrated conservation strategies. Validation through 16 landscape manifestations connects quantitative patterns to observable territorial features. These findings provide a transferable methodological framework for geodiversity assessment in analogous low-relief coastal systems and offer a scientific basis for geoconservation, spatial planning, and the strengthening of the Paisagem das Águas Geopark Project.