The purpose of this study was to examine the effects of high ambient temperature and varying humidity levels on critical power (CP) and physiological variables in cyclists. Twelve male cyclists (age 36 ± 8 years, body height 172 ± 4 cm, body mass 72 ± 10 kg) performed incremental exercise tests (IET) and 3-min all-out tests (3MT) under three environmental conditions: high temperature with high relative humidity (Ht-wet), high temperature with low relative humidity (Ht-dry), and neutral temperature with low relative humidity (Nt-dry). Physiological responses, including maximal oxygen uptake (VO2max) and ventilatory thresholds (VT1, VT2) were assessed relative to power output (wVO2max, wVT1, wVT2). End power (EP), anaerobic work capacity (WEP), and time to exhaustion were also measured. Results showed that VO2max and wVO2max did not differ among conditions, whereas VT2 and wVT1 were highest under the Nt-dry, intermediate under the Ht-dry, and lowest under the Ht-wet condition (p < 0.05). For VT1 and wVT2, values under the Ht-wet condition were significantly lower than under Nt-dry and Ht-dry conditions (p < 0.05). During the 3MT, EP, peak power, and average power were significantly higher under the Nt-dry than under Ht-dry and Ht-wet conditions (p < 0.05). In conclusion, high temperature reduced 3MT performance compared with the neutral condition, with no additional decline between Ht-wet and Ht-dry conditions. In contrast, VT and wVT were most reduced under the Ht-wet condition, indicating greater impairment of submaximal physiological responses under hot-humid conditions. These results suggest that humidity mainly affects threshold-related outcomes rather than 3MT performance. Environmental heat should therefore be considered when applying 3MT-derived variables for training prescription.