Unnilnovium is a chemical element with the temporary name and symbol Uun. Its atomic number is 119. It is the synthetic chemical element with the third-longest name, after unbinilium and unsepttrium, in the periodic table, and one of the four elements predicted by Glenn T. Seaborg early on.

Because of its high atomic number, unnilnovium is generally predicted to be extremely radioactive and to last for only fractions of a second. It is predicted to behave similarly to its lighter homologue thallium, occurring in solids as the +1 oxidation state and in solution as the +1 and +3 states; unlike most group 13 elements, the +3 state would be preferred in aqueous solution.

The name unnilnovium is derived from the Latin words for the numbers one, one, and nine, reflecting its atomic number 119. The name unilennium, as proposed by Seaborg, is an allusion to the millennium celebration of the discovery of the element carbon. The IUPAC rules for naming elements allow the creation of systematic element names for atomic numbers up to 120.