Mendelevium is a transuranium element in the actinide series. It is denoted by the atomic symbol Md. It has an atomic number of 101 and an approximate atomic weight of 258. It is a man-made radioactive element which has no stable nuclides.

The longest-lived isotope of the element, mendelevium-258, has a half life of 56 days. This period is long enough to allow the investigation of some chemical and physical properties. As expected, the element appears to be chemically similar to thulium, the element above it in the periodic table.

Mendelevium was produced in 1955 by the research team of Albert Ghiorso, Gregory Choppin (1927-), Stanley Thompson (1912-), and Glenn T. Seaborg at the University of California at Berkeley. A tiny sample of einsteinium-253 was bombarded with alpha particles in the university's 60-inch cyclotron. The product of the reaction was 17 atoms of an isotope with mass 256 and a half life of 77 minutes.

The research team recommended naming the element mendelevium in honor of Dmitry Mendeleev, the developer of the periodic table.