The purpose of this study was to determine the extent to which measurement latency has significance for the accuracy of heart rate (HR) measured by wearable devices during physical activities. Thirty participants (15 female) aged 18–50 years (mean 24.8, 6.3 SD years) with weight ranging from 53–87 kg (mean 70.5, 9.4 SD kg) participated in this cross-sectional study. Participants completed three submaximal exercise conditions; cycling, treadmill walking/running and rapid arm movements. During the exercises, HR was concurrently measured using the Polar H10 (PL), the Apple Watch 2 (AW) and the Garmin Forerunner 235 (GF). Measurement latency significantly affected the discrepancy in HR measures and measurement latency accounted for 15% (GF) and 2% (AW) of the explained discrepancy in HR between the criterion measure PL and the GF and AW. However, the findings also showed that the magnitude of HR measurement latency differed notably among the two wearable devices and measurement latency was greatly influenced not only by type but also by the intensity of exercise. These findings contribute to our understanding of wearable device HR measures discrepancy and this knowledge may be of assistance when electing to use wearable devices to monitor physical exertion in different contexts.