Scrith, usually written italicized as scrith, is a fictional substance conceived by Larry Niven. It forms the walls and floor of Niven's Ringworld.

Scrith is a milky-gray translucent material impervious to all weapons. The fairly thin layer of scrith that forms the floor of the Ringworld blocks the passage of 40% of the neutrinos that encounter it, equivalent to almost a light year of lead. It also absorbs nearly 100% of all other radiation and subatomic particles and rapidly dissipates heat. The tensile strength of scrith is similar to the strong nuclear force, with the Ringworld foundation only about 30m (100 ft) deep. The physical composition of scrith is unclear, but it appears to share some of the properties of a metal (albeit in a greatly exaggerated form): for instance, the high tensile strength, the ability to conduct heat and the ability to retain an induced magnetic field.

Scrith is said to have been artificially produced through the transmutation of matter. Jaded science fiction readers sometimes refer to it as a form of unobtainium.

Scrith is also nearly frictionless, a fact that gives rise to an episode in the plot of Ringworld that (eventually) tells Louis Wu something very important about Teela Brown: unlike him, she is too lucky to slip on it.