Purpose: To examine the effect of environmental temperature (T A) on performance and physiological responses (eg, body temperature, cardiopulmonary measures) during a high-intensity aerobic interval session. It was hypothesized that power output would be highest in the 13°C condition and lower in the 5°C, 22°C, and 35°C conditions. Methods: Eleven well-trained cyclists randomly completed 4 interval sessions at 5°C, 13°C, 22°C, and 35°C (55% [13%] relative humidity), each involving five 4-min intervals interspersed with 5 min of recovery. During the intervals, power output, core temperature (T C), skin temperature, VO2, and heart rate were recorded. Results: Mean session power output for 13°C (366  W) was not higher than 5°C (363  W; P = 1.00, effect size = 0.085), 22°C (364  W; P = 1.00, effect size = 0.061), or 35°C (352  W; P = .129, effect size = 0.441). The 5th interval of the 35°C condition had a lower power output compared with all other T A. T C was higher in 22°C compared with both 5°C and 13°C (P = .001). VO2 was not significantly different across T A (P = .187). Heart rate was higher in the 4th and 5th intervals of 35°C compared with 5°C and 13°C. Conclusions: This study demonstrates that while mean power outputs for intervals are similar across T A, hot T A (≥35°C) reduces interval power output later in a training session. Well-trained cyclists performing maximal high-intensity aerobic intervals can achieve near-optimal power output over a broader range of T A than previous literature would indicate.