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Thorium ist ein chemisches Element mit dem Elementsymbol Th und der Ordnungszahl 90. Im Periodensystem steht es in der Gruppe der Actinoide (7. Periode, f-Block).
Es ist ein schwach radioaktives, weiches und duktiles Schwermetall, welches das am häufigsten vorkommende radioaktive Element in der Erdkruste ist. Es wird oft aus Monazit gewonnen. Auf der Erde ist es fast ausschließlich als Thorium-232 zu finden, andere Isotope kommen nur in Spuren vor oder sind komplett synthetisch. Früher wurde die Verbindung Thorium(IV)-oxid aufgrund des hohen Schmelzpunktes in Glühstrümpfen verwendet. Aufgrund der Radioaktivität des Thoriums werden gegenwärtig allerdings andere Materialien verwendet. Heutzutage liegt der Fokus eher auf seiner bisher noch experimentellen Verwendung in Flüssigsalzreaktoren, in denen es das Uran ersetzen soll.