The diversity of true water beetles (Coleoptera: Gyrinidae, Haliplidae, Dytiscidae, Helophoridae, Hydrochidae, Hydrophilidae, Hydraenidae, Elmidae, and Dryopidae) in Picos de Europa National Park (Cantabrian Mountains, Spain) was examined. Taking into account historic long-term sampling (all collections from 1882 to the present), a total of 117 species are recorded. Species accumulation models and non-parametric estimators were used to estimate the actual species richness of aquatic Coleoptera occurring in Picos de Europa National Park. Estimates were generated by analyzing both the collector's curve from the long-term sampling and the historic cumulative curve of species recorded from the park since 1882. Values of species richness estimated by different methods range from 127 to 170 species (mean = 148 ± 15 SD). Therefore, it seems that the current inventory has reached a reasonably good level of completeness as estimates indicate that about 80% of the water beetle fauna has already been recorded. The inventory is used to analyze the biological uniqueness of the park and its outstanding level of species richness and endemism (33 Iberian endemic species). Richness and endemism of water beetles in Picos de Europa National Park is compared to those of other national parks in Spain. Finally, the conservation status and threat level is assessed for two Cantabrian endemic species, Deronectes costipennis gignouxi Fery and Brancucci and Ochthebius (Asiobates) cantabricus Balfour-Browne.