Little is known about the separate and combined influences of humidity conditions, sex, and aerobic fitness on heat tolerance in unacclimatized males and females. The purpose of the current study was to describe heat tolerance, in terms of critical WBGT (WBGT(crit)), in unacclimatized young males and females in hot-dry (HD) and warm-humid (WH) environments. Eighteen subjects (9 M/9F; 21 ± 2 yr) were tested during exercise at 30% V̇O(2max) in a controlled environmental chamber. Progressive heat stress exposures were performed with either (1) constant dry-bulb temperature (T(db)) of 34 and 36 °C and increasing ambient water vapor pressure (P(a)) (P(crit) trials; WH); or (2) constant P(a) of 12 and 16 mmHg and increasing T(db) (T(crit) trials; HD). Chamber T(db) and P(a), and subject esophageal temperature (T(es)), were continuously monitored throughout each trial. After a 30-min equilibration period, progressive heat stress continued until subject heat balance could no longer be maintained and a clear rise in T(es) was observed. Absolute WBGT(crit) and WBGT(crit) adjusted to a metabolic rate of 300 W (WBGT(300)), and the difference between WBGT(crit) and occupational exposure limits (OEL; ΔOEL) was assessed. WBGT(crit), WBGT(300), and ΔOEL were higher in WH compared to HD (p < 0.0001) for females but were the same between environments for males (p ≥ 0.21). WBGT(crit) was higher in females compared to males in WH (p < 0.0001) but was similar between sexes in HD (p = 0.44). When controlling for metabolic rate, WBGT(300) and ΔOEL were higher in males compared to females in WH and HD (both p < 0.0001). When controlling for sex, V̇O(2max) was not associated with WBGT(300) or ΔOEL for either sex (r ≤ 0.12, p ≥ 0.49). These findings suggest that WBGT(crit) is higher in females compared to males in WH, but not HD, conditions. Additionally, the WBGT(crit) is lower in females, but not males, in HD compared to WH conditions.