OBJECTIVES: To investigate the efficacy of heat acclimation (HA) in the young (Y(EX)) and elderly (E(EX)) following exercise-HA, and the elderly utilising post-exercise hot water immersion HA (E(HWI)). DESIGN: Cross-sectional study. METHOD: Twenty-six participants (Y(EX): n = 11 aged 22 ± 2 years, E(EX:)n = 8 aged 68 ± 3 years, E(HWI): n = 7 aged 73 ± 3 years) completed two pre-/post-tests, separated by five intervention days. Y(EX) and E(EX) exercised in hot conditions to raise rectal temperature (T(rec)) ≥38.5 °C within 60 min, with this increase maintained for a further 60 min. E(HWI) completed 30 min of cycling in temperate conditions, then 30 min of HWI (40 °C), followed by 30 min seated blanket wrap. Pre- and post-testing comprised 30 min rest, followed by 30 min of cycling exercise (3.5 W·kg(-1) Ḣ(prod),) and a six-minute walk test (6MWT), all in 35 °C, 50% RH. RESULTS: The HA protocols did not elicit different mean heart rate (HR), T(rec), and duration T(rec) ≥ 38.5 °C (p > 0.05) between Y(EX), E(EX), and E(HWI) groups. Resting T(rec), peak skin temperature, systolic and mean arterial pressure, perceived exertion and thermal sensation decreased, and 6MWT distance increased pre- to post-HA (p < 0.05), with no difference between groups. Y(EX) also demonstrated a reduction in resting HR (p < 0.05). No change was observed in peak T(rec) or HR, vascular conductance, sweat rate, or thermal comfort in any group (p > 0.05). CONCLUSIONS: Irrespective of age or intervention, HA induced thermoregulatory, perceptual and exercise performance improvements. Both exercise-HA (E(EX)), and post-exercise HWI (E(HWI)) are considered viable interventions to prepare the elderly for heat stress.