An optical mouse is a computer mouse which uses a light source, typically a light-emitting diode (LED), and a light detector, such as an array of photodiodes, to detect movement relative to a surface. Variations of the optical mouse have largely replaced the older mechanical mouse design, which uses moving parts to sense motion.

The earliest optical mice detected movement on pre-printed mousepad surfaces. Modern optical mice work on most opaque diffusely reflective surfaces like paper, but most of them do not work properly on specularly reflective surfaces like polished stone or transparent surfaces like glass. Optical mice that use dark field illumination can function reliably even on such surfaces.

Though not commonly referred to as optical mice, nearly all mechanical mice tracked movement using LEDs and photodiodes to detect when beams of infrared light did and didn't pass through holes in a pair of incremental rotary encoder wheels (one for left/right, another for forward/back), driven by a rubberized ball. Thus, the primary distinction of "optical mice" is not their use of optics, but their complete lack of moving parts to track mouse movement, instead employing an entirely solid-state system.

The first two optical mice, first demonstrated by two independent inventors in December 1980, had different basic designs: One of these, invented by Steve Kirsch of MIT and Mouse Systems Corporation, used an infrared LED and a four-quadrant infrared sensor to detect grid lines printed with infrared absorbing ink on a special metallic surface. Predictive algorithms in the CPU of the mouse calculated the speed and direction over the grid. The other type, invented by Richard F. Lyon of Xerox, used a 16-pixel visible-light image sensor with integrated motion detection on the same n‑type (5 μm) MOS integrated circuit chip, and tracked the motion of light dots in a dark field of a printed paper or similar mouse pad. The Kirsch and Lyon mouse types had very different behaviors, as the Kirsch mouse used an x-y coordinate system embedded in the pad, and would not work correctly when the pad was rotated, while the Lyon mouse used the x-y coordinate system of the mouse body, as mechanical mice do.

The optical mouse ultimately sold with the Xerox STAR office computer used an inverted sensor chip packaging approach patented by Lisa M. Williams and Robert S. Cherry of the Xerox Microelectronics Center.

The Mouse Systems (Kirsch) design was commercialised and sold in PC compatible form by the company itself alongside variants rebranded for OEM use with Sun Microsystems workstations and by Data General.

Modern surface-independent optical mice work by using an optoelectronic sensor (essentially, a tiny low-resolution video camera) to take successive images of the surface on which the mouse operates. As computing power grew cheaper, it became possible to embed more powerful special-purpose image-processing chips in the mouse itself. This advance enabled the mouse to detect relative motion on a wide variety of surfaces, translating the movement of the mouse into the movement of the cursor and eliminating the need for a special mouse-pad. A surface-independent coherent light optical mouse design was patented by Stephen B. Jackson at Xerox in 1988.

Xerox's inventions were never massively commercially exploited, however, and optical mice would remain elusive in the personal computer market until Microsoft released the IntelliMouse with IntelliEye and IntelliMouse Explorer in 1999. These mice used technology developed by Hewlett-Packard under their Agilent Technologies subsidiary (see below). These mice worked on almost any surface, and represented a welcome improvement over mechanical mice, which would pick up dirt, track capriciously, invite rough handling, and need to be taken apart and cleaned frequently. Other manufacturers soon followed Microsoft's lead, including Apple for their Pro Mouse, using components manufactured by Agilent (once they spun off from HP), and over the next several years mechanical mice became obsolete.