The invention relates to a system for controlling the luminosity of a head-up display (36) of a vehicle, said display (36) being configured to display an image, said system comprising means (32) for assessing luminosity connected to means (34) for determining a luminosity instruction for the display (36). The system is characterised in that said means (32) for assessing luminosity are capable of assessing the external luminosity towards the front of the vehicle and beyond the image displayed on said display (36), and in that said determination means (34) are capable of determining the luminosity instruction in accordance with the external luminosity assessed and of transmitting same to the display (36).