Uracil (often abbreviated to U, chemical formula C4H4N2O2) is a nitrogenous nucleobase, found only in RNA, where it replaces the DNA equivalent of thymine . Within RNA, its monomeric constituent is called uridine monophosphate (UMP), and it is added to the growing RNA chain as a precursor, uridine triphosphate (UTP).

Uracil is a pyrimidine derivative, and it base-pairs with adenine using two hydrogen bonds.