There are described a non-invasive method and a corresponding system for monitoring a heartbeat, which are capable of recognizing a state of normal sinus heartbeat rhythm or a state of probable extra systolic heartbeat rhythm or a state of probable atrial fibrillation, and also capable of discriminating between said states. The method comprises the steps of detecting a sequence of intervals between consecutive heartbeats comprising a predetermined number of intervals, by means of a non- invasive heart rate monitoring device 2； then calculating, on the basis of said sequence of detected intervals, a measure of heart rate variability within the sequence； and then calculating a series of duration differences between two consecutive intervals. The method then includes generating a two-dimensional diagram 10 associated with the sequence of detected intervals, wherein each of the detected intervals between consecutive heartbeats is characterized by the duration and by the duration difference with respect to the immediately preceding interval, and is represented in the diagram by a point on a grid having, as the first Cartesian coordinate, the duration of the interval and, as the second Cartesian coordinate, the duration difference between two consecutive intervals. The method then includes the steps of detecting the number of non-empty cells (NEC), in the aforesaid diagram, containing at least one point representative of an interval between consecutive heartbeats； comparing the calculated measure of heart rate variability, within said sequence, with a first threshold； and comparing the number of non-empty cells (NEC) in the diagram with a second threshold. The method finally comprises the steps of recognizing a state of normal sinus heartbeat rhythm, if the calculated measure of heart rate variability, in the sequence, is lower than the first threshold, and the number of non-empty cells (NEC) is lower than the second threshold； recognizing a state of probable atrial fibri
