Atlantic stingrays, Dasyatis sabina, inhabit shallow bays subject to rapid temperature changes, yet little is known about their thermal tolerance strategies. We quantified critical thermal maxima (CTMaxima) and minima (CTMinima) of Atlantic stingrays from St. Joseph's Bay, Florida. Spiracle contraction cessation for more than one minute was the experimental endpoint. Laboratory-acclimated fish held at 10.8, 20.5, or 35.1 C exhibited CTMaxima of 35.7, 39.3, or 43.2 C, and CTMinima of 0.8, 4.8, or 10.8 C, respectively. Regression analysis revealed that CTMaxima increased by 0.31 C and CTMinima decreased by 0.41 C for every 1.0 C increase in acclimation temperature. Atlantic stingrays exposed to diel thermoperiods of 34.0–37.3 C and 7.0–11.0 C had CTMaxima of 43.1 and 33.4 C and CTMinima of 9.6 and 0.3 C, respectively, suggesting that fish acclimated to low cycle temperatures. A thermal tolerance polygon produced from laboratory data had an area of 978 C², the third largest ever measured in a fish. Seasonally acclimatized fish achieved their lowest respective CTMaxima and CTMinima of 37.3 and 2.5 C in March and highest values of 41.8 and 6.6 C during July. Low extreme temperatures in St. Joseph's Bay came within 0.3 C of the fishes' CTMin; however, extreme high temperatures came no closer than 6.5 C of the fishes' CTMax. Similar to laboratory responses, seasonally acclimatized Atlantic stingrays tuned their physiology to the low cycle temperatures that were most threatening to continued survival.