Background and objective <p>Heat acclimation is a common strategy for athlete preparing to compete in the heat. It is generally obtained by exercising in a hot environment, (e.g. environmental chambers) but passive exposure to hot water has also been shown to induce acclimation. Thus, combining exercise in hot water could be an innovative method to enhance performance in hot environments.</p> Methods <p>In this prospective randomized controlled trial, two groups of 12 participants (22 males/2 females) underwent 10 aerobic training sessions on an underwater bike in either hot (35&#xa0;°C—hot water acclimation, HWA) or temperate (25&#xa0;°C, control, CON) water. Two to three days before and after intervention participants performed a 30-min time trial, in a hot and humid environment (T°: 38.4 ± 1.4&#xa0;°C, Relative Humidity: 45.3 ± 6.9%), as well as a cardiac echography, a blood analysis and a cardiopulmonary exercise test (V̇O_2max) in temperate ambient conditions.</p> Results <p>The distance covered during the time trial in the heat increased depending on group (<i>p</i> = 0.024) with an increase of 1.1[+ 0.7, + 1.4]km in HWA (<i>p</i> &lt; .001) and 0.5[+ 0.1, + 0.8]km in CON (<i>p</i> = 0.009).</p> <p>As well as power output (<i>p</i> = 0.096) improved more in the HWA vs. CON group (+ 27W in HWA and + 13W in CON). However, no difference between groups was observed regarding changes in physiological parameters during the time trial in the heat (core and skin temperature, sweat rate) or across the tests and measures in temperate environments (plasma volume, heart rate, V̇O_2max).</p> Conclusion <p>Exercise on a bike in hot water improves performance in hot and humid conditions but does not induce change physiological parameters in ambient conditions.</p> <p>ClinicalTrials.gov Identifier: NCT05727774.</p>