Zoological collections are the main repositories of biodiversity for specific regions or taxa. However, they often lack promotion and consequently remain largely unknown to the general public or even for specialists. Moreover, many of these collections are at risk of deterioration and, in many cases, lack taxonomic review. Founded in 1771, the Museo Nacional de Ciencias Naturales (MNCN; Madrid, Spain) is one of the oldest Natural History Museums in the world and one of the most important scientific research institutions in the field of Natural Sciences in Europe. The review conducted in this study resulted in an annotated catalogue of marine Heterotremata crabs, documenting 83 species across 21 families from the MNCN collection. It was found that only 21.9% of the specimens had been correctly identified, while 78.1% were either unidentified, identified only to the family level, or misidentified. This highlights a significant loss of biodiversity information and missed opportunities for identifying new species, as many specimens had been inadequately identified for up to 175 years. The review also expanded the known geographic distribution of several species, with the first-time recordings in regions such as Mauritian, Equatorial Guinean, Moroccan, Ghanaian, and São Tomé and Príncipe waters. The study emphasizes the importance of access to zoological collections and collaboration with specialists to enhance our understanding of biodiversity.