Thorium (Th) is a metallic chemical element with atomic number 90. Almost all thorium found in nature is of the isotope ²³²Th, with 90 protons and 142 neutrons. Thorium is one of the more abundant heavy elements.

While nuclear reactors most commonly run on uranium, it is also possible to use thorium as the fuel. One advantage of this is that thorium is more abundant. For example, India has the largest thorium supply in the world, but has relatively little uranium, and as a result India is investing into thorium-based nuclear power plants.