Climate-induced warming can impose thermal stress on organisms, impairing their physiology and further impacting ecosystem structure and functioning. Despite the significance of climate change, the response to warming in most species is poorly understood. The majority of thermotolerance research focuses on single aspects of an organism’s thermal performance, thereby failing to predict overall functioning with respect to future climate predictions. We characterised the response to warming of the New Zealand amphipod Melita inaequistylis. In coastal benthic ecosystems amphipods are an important link between primary producers and higher consumers, performing an essential role in coastal ecosystems. We investigated M. inaequistylis’ thermotolerance, temperature selection, fecundity, metabolism, growth, and adaptive capacity under a range of different temperatures, providing a broad understanding of their functioning in current and future climates.