The biology of cave‐dwelling species is a fascinating area, yet it is understudied due to logistic constraints, especially in regard to aquatic organisms. The olm (Proteus anguinus) is the largest known troglobiont vertebrate, showing extreme life‐history characteristics such as a possible lifespan of over 100 years and a reproductive cycle of over 12 years. However, most studies carried out on the species to date are based on laboratory studies, resulting in a severe lack of ecological data from natural populations studied in their original habitat. We applied a capture–mark–recapture approach on an Eastern Herzegovinian population for eight years to reveal its spatial strategy and general movement patterns. We found that P. anguinus is sedentary, can often be found within a few square metres over several years, and their moving distance is uncorrelated with the time elapsed between recaptures. Previous laboratory studies constrained to confined space and artificial environments have already suggested site fidelity and our results from a natural population confirm this. The low reproductive activity of the species together with the reported extreme site fidelity makes this top predator of aquatic cave communities highly vulnerable and a sensitive bio‐indicator of habitat‐changing human activities. This is the first capture–mark–recapture study on a natural population of the obligate aquatic cave‐dwelling olm (Proteus anguinus), known for its extreme life‐history characteristics (lifespan over 100 years, reproductive cycle over 10 years). We observed extreme site fidelity; individuals moved typically less than 15 m over years.