A disco ball, mirror ball, or ball mirror is a roughly spherical structure that reflects light directed at it in many directions, producing a complicated and hard-to-anticipate display. Its surface consists of hundreds or thousands of facets, nearly all of approximately the same shape and size, and each having a mirrored outer surface. Usually it is mounted well above the heads of the people present, suspended from a device that causes it to rotate steadily on a vertical axis so that stationary viewers experience beams of light flashing over them.

In the 1970s, these devices were a standard accompaniment to disco music, and by the end of the 20^th century, the name "disco ball" had largely eclipsed the earlier names that had been applied to, for example, the one that can be seen in the 1942 movie Casablanca. The patterns of scattered light from a disco ball are difficult to recreate by other means, but the decline in popularity of disco in the United States, and the availability of electronics of cheaper and far more flexible lighting systems, have since turned disco balls into rarer sights.

Miniature disco balls are sold as novelties and used for a number of decorative purposes, including dangling from the rear-view mirror of an automobile.

With the appearance of infrared networks, disco balls have found a new application, as a method of dispersing the infrared signals.