Commercially available wrist-worn heart rate (HR) monitors have become increasingly popular.However, the accuracy of the devices across a range of exercise intensities is largely unknown. This study investigated the accuracy of four wrist-worn HR monitors (Apple Watch Series 1, Fitbit Charge, TomTom Touch, and Mio Fuse). Eighteen adults completed three trials on a cycle ergometer wearing a chest-worn HR monitor (Polar). Trial 1 established the HR-power output relationship, and resting and maximum HR. In trials 2 and 3, participants were fitted to an electrocardiogram (ECG) and completed a step test consisting of 5 x 3 minute stages at 40 -80% of HR reserve (determined in trial 1) whilst wearing two wrist-worn HR monitors. Relative to ECG, there were no differences in HR between the devices during exercise (p = 0.239), and no device × exercise intensity interaction (p = 0.370). There were no instances where ECG and Polar data differed by ≥ 5 b·min -1 . Conversely, there were two instances (2.2%) with the Apple, four (4.4%) with the Mio, 10 (11.1%) with the TomTom, and 19 (21.1%) with the Fitbit. A chest-worn HR monitor offers greater accuracy compared to wrist-worn devices.