Dubnium (Db), element 104, is a synthetic, radioactive, metallic element and the first of the transactinide elements. The longest-lived of its 10 isotopes has a half-life of 70 seconds. Dubnium forms a volatile compound with chlorine.

The University of California at Berkeley claimed to have synthesized the element in 1969, but the Joint Institute for Nuclear Research in Dubna, Russia, claimed production five years later. A name compromise of "unnilquodium" (symbol Unq) proved unappealing. "Dubnium" was adopted in 1995 over the proposed US name of "rutherfordium" (symbol Rf), honoring British physicist Ernest Rutherford, and Russia's proposal of "kurchatovium" (symbol Ku) for Soviet physicist Igor Kurchatov (1903–1960). Rutherfordium has now become element 106.