ABSTRACT Accurate measures of heart rate (HR) during rehabilitation and sporting activities are important for precise exercise prescription to maintain or increase capacity. Wrist-worn activity monitors utilizing photoplethysmography technology (PPG) to configure HR show discrepant findings regarding validity depending on the type and intensity of exercises measured, and no previous study has yet investigated the accuracy during running at speeds exceeding 9.6 km/h. The purpose of the study was to assess the accuracy of the Garmin Forerunner 235 (GF), at different exercises at various intensities. Twenty-nine participants participated in the study. HR was measured with the (GF) during rest and three submaximal exercise conditions; cycling, treadmill walking, running and rapid arm movements. The GF had high agreement with the PL during rest (r = 0.997) cycling at 150 W (Rho = 0.889), treadmill running at 8.7 km/h (r = 0.906) and 12.1 km/h (r = 0.845) and rapid arm movements (r = 0.928, r = 0.745) but a low agreement during cycling at 50 W (Rho = 0.269) and 100W (Rho = 0.462) and treadmill walking at 4.8 km/h (r = 0.481). The results varied across exercise conditions and intensities and although the GF provided accurate measurements of HR during rest, cycling at 150W, treadmill running, and rapid arm movement measurement latency may potentially affect application.