Summary The critical thermal maximum (CTmax) and the associated hematological response of juvenile (~145 g, n = 8 for both species) Atlantic Acipenser oxyrinchus and shortnose Acipenser brevirostrum sturgeons acclimated to 15°C were determined using a heating rate of 8°C h−1. The critical thermal maximum averaged 30.8°C and 31.6°C for Atlantic and shortnose sturgeon, respectively, and values fell within the range noted for other sturgeon species. Oxygen‐carrying capacity (hemoglobin and hematocrit) measures were generally unaffected by thermal stress. Plasma lactate levels increased from 0.5 mm to 4 mm following temperature stress in both species. Both plasma glucose and potassium levels increased following CTmax, however, these levels were about double in the shortnose sturgeon. Lastly, plasma sodium and chloride levels were significantly depressed (by more than 10%) following thermal stress in shortnose sturgeon, whereas only chloride levels decreased in Atlantic sturgeon. Taken together, while CTmax values were similar, thermal stress resulted in different hematological profiles; these differences are consistent when compared to other stressors, and may be related to the phylogenetic position and thus could reflect the evolutionary history of these two species.