The aim of this study was to investigate the effect of repeated passive heat exposure (i.e. acclimation) on muscle contractility in humans. Fourteen non-heat acclimated males completed two trials including electrically evoked twitches and voluntary contractions in thermoneutral conditions (COOL: 24ºC, 40% RH) and hot ambient conditions in the hyperthermic state (HOT: 44-50ºC, 50% RH) on consecutive days in a counterbalanced order. Rectal temperature was ~36.5ºC in COOL and was maintained at ~39ºC throughout HOT. Both trials were repeated after 11 days of passive heat acclimation (1 h per day, 48-50ºC, 50% RH). Heat acclimation decreased core temperature in COOL (-0.2ºC, p<0.05), increased the time required to reach 39ºC in HOT (+9min, p<0.05) and increased sweat rate in HOT (+0.7 l.h-1, p<0.05). Moreover, passive heat acclimation improved skeletal muscle contractility as evidenced by an increase in evoked peak twitch amplitude both in COOL (20.5±3.6 vs. 22.0±4.0 N.m) and HOT (20.5±4.7 vs. 22.0±4.0 N.m) (+9%, p<0.05). Maximal voluntary torque production was also increased both in COOL (145±42 vs. 161±36 N.m) and HOT (125±36 vs. 145±30 N.m) (+17%, p<0.05), despite voluntary activation remaining unchanged. Furthermore, the slope of the relative torque/EMG linear relationship was improved post-acclimation (p<0.05). These adjustments demonstrate that passive heat acclimation improves skeletal muscle contractile function during electrically evoked and voluntary muscle contractions of different intensities both in COOL and HOT. These results suggest that repeated heat exposure may have important implications to passively maintain or even improve muscle function in a variety of performance and clinical settings.