The electrocardiogram (ECG) serves as the gold standard of heart rate (HR) monitoring but is rarely used outside of a clinical environment. Newly developed wearable technology is more usable outside of a clinical setting, but has not been validated against this gold standard. PURPOSE: The purpose of this study is to determine the validity of a wrist-mounted photoplethysmography (PPM) device used for measuring HR during incremental treadmill exercise using ECG as the criterion HR measurement. METHODS: Twenty-two subjects (13 men, 9 women; 35.8 ± 6.3 yr, 14.6 ± 7.5 % body fat, VO₂max: 55.5 ± 0.49 ml·kg^-1·min^-1) performed a Bruce treadmill protocol graded exercise test. HR was recorded at rest and at the end of each minute with the Mio Alpha PPM device and ECG simultaneously. HR was compared between methods across the entire testing session (rest and exercise values) and separately for each exercise test stage using paired-samples t-tests and the Bonferroni correction. Validity coefficients were determined using the Pearson correlation. RESULTS: HR across the entire intensity range (rest to maximal exercise) exhibited a significant correlation between methods (r = 0.97, p < 0.001) and was similar between ECG and Mio after the Bonferroni correction was applied, requiring p < 0.008 (overall mean HR: ECG = 124 ± 39 b·min^-1, Mio = 123 ± 37 b·min^-1, t₃₅₉ = -2.504, p = 0.013). Significant correlations were observed at rest and each exercise test stage, with r values ranging from 0.67 to 0.96 (all p < 0.001). HR was similar between methods at rest and each exercise test stage except for stage 4 (ECG = 168 ± 13 b·min^-1, Mio = 164 ± 14 b·min^-1, p = 0.004). CONCLUSIONS: Correlational analyses indicated a strong agreement between HR methods overall, as well as individually at rest and during each exercise test stage. However, mean comparisons observed a significant difference between methods at stage 4 and mean differences between methods became larger as intensity increased. From a practical standpoint, the mean difference between methods did not exceed 3 b·min^-1 except for stage 5. Therefore, a PPM device seems valid for HR monitoring during low-to-moderate-intensity but not high-intensity treadmill exercise.