The isotopic composition of many elements varies across both land and ocean surfaces in a predictable fashion. These stable-isotope ratios are transferred into animal tissues, potentially providing a powerful natural geospatial tag. To date, most studies using stable isotopes as geolocators in marine settings have focussed on mammals and seabirds conducting large ocean-basin scale migrations. An increasing understanding of isotopic variation in the marine environment, and improved sampling and analytical techniques, however, means that stable isotopes now hold genuine promise as a natural geolocation tag in marine fishes. Here, the theoretical background underpinning the use of stable isotopes of C, N and O in otolith, scale and muscle tissues as geolocation tools in the marine environment is reviewed, and examples of their applications are provided.