A display device, a display system, a mobile object, a display-brightness control method, and a recording medium storing program code. The display device and the display-brightness control method include obtaining brightness data indicating brightness in a peripheral area at prescribed time intervals, the peripheral area including at least some of a display area that serves as a background of display data, detecting a change in brightness in the peripheral area based on the brightness data obtained in the obtaining, determining display-brightness data that indicates display brightness when the change in brightness is detected, such that length of delay time before display brightness of the display data is made to correspond to the brightness of the peripheral area is equal to or shorter than a permissible delay time, and displaying the display data using the display-brightness data determined in the determining.