The severity and frequency of marine heatwaves (MHWs) have increased dramatically across the globe, particularly in the last decade. In ectotherms like fishes, temperature directly influences oxygen transport, leading to changes in oxygen consumption (i.e., metabolic rates). The effects of temperature on metabolism have consequences for organismal function, including swimming performance. Performance is expected to decline as metabolic demands accumulate at temperatures beyond a fish’s optimum, though declines may be mitigated through various means, including plastic responses. Rapid changes in temperature, exemplified through MHWs, may outpace an individual’s acclimation capacity, leaving them more vulnerable to detrimental thermal effects. We investigated how heatwave exposure (i.e., +2°C and +4°C above normal summer temperature) over a five-day period affects the metabolic rate and swimming performance of a common nearshore marine fish species, the shiner perch (Cymatogaster aggregata), on San Juan Island, Washington. Using a swim tunnel respirometer, we measured standard and maximum metabolic rate (SMR and MMR, respectively), as well as critical swimming speed (Ucrit) and optimal swimming speed (Uopt). SMR was highest with moderate heatwave exposure (+2°C), while MMR was highly variable within treatments and increased slightly at the highest treatment temperature (+4°C). Ucrit did not vary across temperatures, but Uopt did decline with increasing temperatures. These findings provide insight into how this species may respond to future projected marine heatwaves, and also highlight the need for further studies focusing on metabolic performance across a range of relevant temperatures and acclimation periods.