What is disclosed is a system and method for determining whether a subject is in atrial fibrillation. A video is received of a region of exposed skin of a subject. The video is acquired of a region where a videoplethysmographic (VPG) signal can be registered by at least one imaging channel of a video imaging device. For each batch of image frames, pixels associated with the region of exposed skin are isolated and processed to obtain a time-series signal. A VPG signal is extracted from the time-series signal. The power spectral density (PSD) is computed across all frequencies within the VPG signal. A pulse harmonic strength (PHS) is calculated for this VPG signal. The pulse harmonic strength is compared to a discrimination threshold, defined herein. A determination is made whether the subject in the video is in atrial fibrillation or in normal sinus rhythm.