The Mediterranean Sea faces escalating environmental pressures from the unsustainable exploitation of resources and insufficient management. Marine protected areas (MPAs), such as the Parc Natural del Montgrí, les Illes Medes i el Baix Ter (PNMMBT) on Spain’s Costa Brava, serve as critical refuges for biodiversity conservation. Between 2010 and 2019, we conducted systematic visual surveys of sea slugs (Mollusca: Heterobranchia) within the PNMMBT, documenting 8289 individuals representing 98 species: 76 Nudibranchia, 10 Sacoglossa, 5 Aplysiida, 3 Pleurobranchida, 2 Umbraculida, 1 Runcinida, and 1 Cephalaspidea. Temporal, spatial, and bathymetric distribution analyses revealed a steady increase in species richness over time, suggesting the MPA’s ecosystems appear to remain in relatively good condition. To further refine our assessment, we incorporated citizen science records from the MINKA platform, expanding the documented diversity to 141 species including 99 Nudibranchia, 12 Sacoglossa, 5 Aplysiida, 6 Pleurobranchida, 2 Umbraculida, 4 Runcinida, 10 Cephalaspidea, 1 Acteonimorpha, 1 Pteropoda, and 1 Ringiculimorpha. This integrated approach highlights the value and complementarity of structured scientific surveys and community-contributed data in biodiversity monitoring. Citizen science fosters public engagement in research and provides scalable, cost-effective data that would otherwise be logistically unfeasible to obtain. By bridging these methodologies, we demonstrate how participatory science enhances the detection of species distributions, tracks ecological shifts, and bolsters conservation strategies in the current era of global biodiversity decline.