A device (1) for determining a personalized sun exposure program has a portable casing (2) housing a processing unit (4) associated with a memory (5); the processing unit (4) is connected to a data-entry keyboard (9), to sun sensors (12), and to a light source (15) associated with a photometric detector (16) for measuring the light emitted by the light source (15) and reflected by a user's skin surface; the processing unit (4) calculates an intensity value (I), representing local sun conditions, on the basis of the measurement supplied by the sensors (12), and calculates a reflection value (R D ), representing the user's phototype and skin condition, on the basis of the measurement supplied by the detector (16); the processing unit then supplies a personalized sun exposure program as a function of the reflection value (R D ) and the intensity value (I); and the program indicates one or more exposure periods and, for each period, the time of day, the exposure time, and the sun protection factor to be used in that period.