Underwater (UW) running provides a low load-bearing form of supplementary training that can be used for recovery and rehabilitation while maintaining cardiovascular fitness. Whether it elicits a cardiorespiratory training stimulus comparable to that of land-based running is seemingly unclear. The purpose of this study was to compare cardiorespiratory responses between underwater treadmill running and land-based running. Fourteen male triathletes completed trials at maximal and submaximal workloads for each of three conditions: running on an underwater treadmill withAQx® water running shoes, running on an underwater treadmill barefoot, and running on a land-based treadmill. No differences between groups were found for measures of oxygen consumption (VO2), rating of perceived exertion (RPE), or respiratory exchange ratio (RER) across modalities for maximal trials; however, heart rate (HR) was greater during land-based running than underwater treadmill running. No group differences were found for HR, RPE, and RER across modalities during submaximal trials; however, VO2 was significantly greater during land-based running than underwater treadmill running. We concluded that the cardiorespiratory training stimulus during underwater treadmill running was comparable to that of land-based running at maximal exertion levels, with the exception of HR, and therefore could be an effective form of supplemental training during rehabilitation.At submaximal levels, underwater treadmill running elicited a less rigorous training stimulus than land-based running in terms of VO2 and therefore is a less effective form of supplemental training.