Promethium ( /prɵˈmiːθiəm/ pro-MEE-thee-əm), originally prometheum, is a chemical element with the symbol Pm and atomic number 61. It is notable for being the only element besides technetium all of whose isotopes are radioactive but which is followed in the periodic table by chemical elements with stable isotopes. Chemically, promethium is a lanthanide, which forms salts when combined with other elements; the compounds, however, have not been fully studied. Promethium shows only one stable oxidation state of +3; however, a few +2 compounds may be capable of existence.

In 1902, Bohuslav Brauner suggested there was an element with properties intermediate between those of the known elements neodymium (60) and samarium (62); this was confirmed in 1914 by Henry Moseley who, having measured the atomic numbers of all the elements then known, found there was no element with atomic number 61. In 1926, an Italian and an American group claimed to have isolated a sample of element 61; both "discoveries" were soon proven to be false. In 1938, during a nuclear experiment conducted at Ohio State University, a few radioactive nuclides were produced that certainly were not radioisotopes of neodymium or samarium, but there was a lack of chemical proof that element 61 was produced, and the discovery was not generally recognized. Promethium was first produced and characterized at Oak Ridge National Laboratory in 1945 by the separation and analysis of the fission products of uranium fuel irradiated in a graphite reactor. The discoverers proposed the name "prometheum" (the spelling was subsequently changed), derived from Prometheus, the Titan in Greek mythology, who stole fire from Mount Olympus and brought it down to mankind, to symbolize "both the daring and the possible misuse of mankind's intellect." However, a sample of the metal was made only in 1963.