This study was performed to assess the validity of the MyWellness Key (MWK) accelerometer during a treadmill-based protocol. The identification of different exercise intensities is imperative to objectively measure time spent at a specified exercise intensity. Thirty subjects, 15 men and 15 women (age = 24.5 ± 2.6 years; body mass index = 22.5 ± 2.5 kg·m^-1), participated in a 4-phase treadmill protocol (5 minutes each one) using three different walking velocities (3, 4.5, and 6 km·h^-1) and run (8 km·h^-1) while outfitted with a MWK uniaxial accelerometer. Oxygen consumption was measured by indirect calorimetry (IC_VO2). Results: The relationship between VO₂ predicted from MWK (MWK_VO2) and oxygen consumption (VO₂ (IC_VO2)), yielded a high and significant correlation (r = 0. 944; p < 0.001) with standard error of estimate (SEE) = 2.42 mL·kg^-1·min^-1. The average differences between the two methods (MWK_VO2 - IC_VO2) were -0.79 (-8. 8% at 3 km·h^-1), -0.02 (-0.2% at 4.5 km·h^-1), 0.51 (3.3% at 6 km·h^-1) and -0.74 (-2.7% at 8 km·h^-1) ml·kg^-1·min^-1. Only the 3 km·h^-1 speed showed a difference when compared to the criterion measure (p < 0.001). Bland and Altman analysis revealed less than a 1 MET difference in the mean at each point estimate and relatively tight distribution with the standard errors, especially with the 2 moderate walking speeds. Conclusions: We found a high correlation between oxygen utilization and the MWK with low standard errors estimates. This indicates that this accelerometer can be used to identify exercise intensities that are related to walking and running.