An ultra-thin watch is a watch whose size and thickness have been reduced to an absolute minimum. Watchmakers consider the bulk of the work to be in the miniaturization of the parts, rather than in actual complicated watch work.

In 1903, JLC designed a 1.38 mm thick pocket watch movement.
In 1925, AP made a 1.32 mm pocket watch movement, Blancpain made a 1.75 mm thick wristwatch.