VO2 max is a measure of cardiorespiratory fitness and a key indicator of overall health. It is predictive of cardiovascular events and shows a strong inverse association with all-cause mortality. Increased cardiorespiratory fitness is associated with reductions in coronary artery disease, diabetes and cancer. Apple Watch offers a less resource-intensive and more feasible alternative to the gold standard assessment for VO2 max, indirect calorimetry, but the accuracy of its measurements remains uncertain. This study aimed to assess the validity of VO2 max estimates from Apple Watch in comparison to indirect calorimetry. Thirty participants wore an Apple Watch for 5-10 days to generate a VO2 max estimate. Subsequently, they underwent a maximal exercise treadmill test in accordance with the modified Åstrand protocol. The agreement between measurements from Apple Watch and indirect calorimetry was assessed using Bland-Altman analysis, mean absolute percentage error (MAPE), and mean absolute error (MAE). Overall, Apple Watch underestimated VO2 max, with a mean difference of 6.07 mL/kg/min (95% CI 3.77-8.38). Limits of agreement indicated variability between measurement methods (lower -6.11 mL/kg/min; upper 18.26 mL/kg/min). MAPE was calculated as 13.31% (95% CI 10.01-16.61), and MAE was 6.92 mL/kg/min (95% CI 4.89-8.94). These findings indicate that Apple Watch VO2 max estimates require further refinement prior to clinical implementation. However, further consideration of Apple Watch as an alternative to conventional VO2 max prediction from submaximal exercise is warranted, given its practical utility.