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Didymium (Greek: twin element) is a mixture of the elements praseodymium and neodymium and used especially for coloring glass for optical filters.

Didymium was discovered by Carl Mosander in 1841 and was so named because it is very similar to lanthanum, with which it was found. Mosander wrongly believed didymium to be an element, under the impression that "ceria" (sometimes called cerite) isolated by Jöns Jakob Berzelius in 1803 was really a mixture of cerium, lanthanum and didymium. He was right about lanthanum's being an element, but not about didymium. Mosander did as well as could be expected at the time, since spectroscopy had not yet been invented. His three "elements" accounted for at least 95% of the rare earths in the original cerite from Bastnäs, Sweden. Didymium had not been difficult to find, since it was providing the pinkish tinge to the salts of ceria when in trivalent form. During the period when didymium was believed to be an element, the symbol Di was used for it. In the illustration of Mendeleev's first attempt at a periodic table, shown on the right, it will be noted that the atomic weights assigned to the various lanthanides, including didymium, reflect the original belief that they were divalent. Their actual trivalency meant that his atomic weights for them were only about 67% of their true values.

In 1874, Per Teodor Cleve deduced that didymium was made up of at least two elements. In 1879, Lecoq de Boisbaudran succeeded in isolating samarium, from didymium contained in North Carolinian Samarskite. Then in 1885, Carl Auer von Welsbach succeeded in separating salts of the last two component elements, which were soon named praseodymium and neodymium. The method used was a fractional crystallization of the double ammonium nitrates from a solution of nitric acid. Welsbach had decided to name his two new elements "praseodidymium" and "neodidymium" ("green didymium" and "new didymium"), but one syllable was soon dropped from each name. The name of "didymium" lived on in untruncated version, partly due to the use in glassblower's goggles. The name "didymium" also lived on in mineralogical texts.

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