The Clock of the Long Now, also called the 10,000-year clock, is a mechanical clock under construction that is designed to keep time for 10,000 years. It is being built by the Long Now Foundation. A two-meter prototype is on display at the Science Museum in London. As of June 2018, two more prototypes are on display at The Long Now Museum & Store at Fort Mason Center in San Francisco.

The project was conceived by Danny Hillis in 1986. The first prototype of the clock began working on December 31, 1999, just in time to display the transition to the year 2000. At midnight on New Year's Eve, the date indicator changed from 01999 to 02000, and the chime struck twice.

The manufacture and site construction of the first full-scale prototype clock is being funded by Jeff Bezos's Bezos Expeditions, with $42 million, and is on land which Bezos owns in Texas.