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 Long Now Foundation, established in 1996, is an American non-profit organization based in San Francisco that seeks to start and promote a long-term cultural institution. It aims to provide a counterpoint to what it views as today's "faster/cheaper" mindset and to promote "slower/better" thinking. The Long Now Foundation hopes to "creatively foster responsibility" in the framework of the next 10,000 years. In a manner somewhat similar to the Holocene calendar, the foundation uses 5-digit dates to address the Year 10,000 problem. The organization's logo is X, a capital X with an overline, a representation of 10,000 in Roman numerals.