Field research assessing the effects of both intrinsic and extrinsic factors on animal daily rhythms has been rare, particularly in carnivores and other medium-to-large mammals in hot environments (e.g., Mediterranean regions). This is a crucial knowledge gap, hindering the accurate prediction of how animals might react to widely-anticipated global environmental changes. To assess the behavioral plasticity of Mediterranean semiaquatic mammals in coping with harsh climatic conditions, we investigated the effects of intrinsic and extrinsic factors on the daily rhythms of a Mediterranean population of radio-tracked Eurasian otters (Lutra lutra). Multiple components cosinor population models revealed markedly nocturnal activity, affected by seasonality and air temperature. In particular, otters lowered their daylight activity and increased their nighttime activity during the dry season, being generally less active under higher air temperatures. Other extrinsic factors affecting otters’ daily rhythms were moon phase, habitat type and wind, whereas differences in activity patterns according to intrinsic factors were mainly related to males’ ranging behavior and females’ reproductive status. This study provided detailed and population-mean data on Lutra lutra daily rhythms, including some insights into inter-individual variation. More importantly, it showed that otters are well adapted to the dry and wet season-cycle typical of Mediterranean regions, while highlighting their potential vulnerability to increasing air temperatures.