Temperature is known to impact host-parasite interactions in various ways. Such effects are often regarded as the consequence of the increased metabolism of parasites with increasing temperature. However, the effect of temperature on hosts’ immune system could also be a determinant. Here we assessed the influence of temperature on the immunocompetence of the crustacean amphipod Gammarus pulex. Amphipods play a key ecological role in freshwater ecosystems that can be altered by several parasites. We investigated the consequences of three weeks of acclimatization at four temperatures (from 9 °C to 17 °C) on different immunological parameters. Temperature influenced both hemocyte concentration and active phenoloxidase enzymatic activity, with lower values at intermediate temperatures, while total phenoloxidase activity was not affected. In addition, the ability of gammarids to clear a bacterial infection was at the highest at intermediate temperatures. These results suggest a dysregulation of the immune system of gammarids in response to stress induced by extreme temperature.