Fermium is an actinide series transuranium element denoted by the atomic symbol Fm. Its atomic number is 100 and the atomic weight of its most stable isotope is 257. Fermium is a radioactive element which has sixteen known isotopes. It is named for Enrico Fermi, Italian physicist who made great contributions to the study of nuclear reactions.

The first evidence of the existence of fermium was found in the debris produced by the first thermonuclear (fusion) bomb test at Bikini Atoll in 1952. But this information was not published. Meanwhile, fermium was also discovered independently in 1954 by a research team at the Nobel Institute of Physics in Stockholm. The element was produced there by bombarding uranium-238 with oxygen-16 ions.

The longest-lived isotope of the element is fermium-257, with a half life of 80 days. In comparison, the isotope discovered in 1952, fermium-255, had a half life of 20.1 hours. Studies of the element usually involve no more than a few atoms at a time. The chemical properties of fermium remain largely unknown however, they have been investigated in an aqueous medium. Fermium has been used to produce heavier elements such as nobelium.