The present invention provides a method for detecting the start of a dive for a portable electronic device (1) comprising a pressure sensor (5) for measuring the value of the ambient pressure (P) as well as electronic circuits (2) for processing the pressure measurement results comprising a time base (4) and at least one memory zone (7, 8). The detection method updates the reference pressure value (Pref) on the basis of ambient pressure (P) measurements, this value being used to detect the start of a dive. In certain particular conditions, the method according to the present invention suspects the start of a dive, and backs up the value (Psauv) of the reference pressure (Pref) stored before updating. If the start of a dive is confirmed after having been suspected, the reference pressure value (Pref) stored is replaced by the backed up value (Psauv) upon activation of the dive mode.