AIMS: We aimed to investigate the concordance between heart rate variability (HRV) derived from the photoplethysmographic (PPG) signal of a commercially available smartwatch compared with the gold-standard high-resolution electrocardiogram (ECG)-derived HRV in patients with cardiovascular disease. METHODS AND RESULTS: We prospectively enrolled 104 survivors of acute ST-elevation myocardial infarction, 129 patients after an ischaemic stroke, and 30 controls. All subjects underwent simultaneous recording of a smartwatch (Garmin vivoactive 4; Garmin Ltd, Olathe, KS, USA)-derived PPG signal and a high-resolution (1000 Hz) ECG for 30 min under standardized conditions. HRV measures in time and frequency domain, non-linear measures, as well as deceleration capacity (DC) were calculated according to previously published technologies from both signals. Lin\'s concordance correlation coefficient (rho(c)) between smartwatch-derived and ECG-based HRV markers was used as a measure of diagnostic accuracy. A very high concordance within the whole study cohort was observed for the mean heart rate (rho(c) = 0.9998), standard deviation of the averages of normal-to-normal (NN) intervals in all 5min segments (SDANN; rho(c) = 0.9617), and very low frequency power (VLF power; rho(c) = 0.9613). In contrast, detrended fluctuation analysis (DF-alpha1; rho(c) = 0.5919) and the square mean root of the sum of squares of adjacent NN-interval differences (rMSSD; rho(c) = 0.6617) showed only moderate concordance. CONCLUSION: Smartwatch-derived HRV provides a practical alternative with excellent accuracy compared with ECG-based HRV for global markers and those characterizing lower frequency components. However, caution is warranted with HRV markers that predominantly assess short-term variability.