Cyclopentadienone is an organic compound with molecular formula C<sub>5</sub>H<sub>4</sub>O. The parent cyclopentadienone is rarely encountered, because it rapidly dimerizes. Many substituted derivatives are known, notably tetraphenylcyclopentadienone. Such compounds are used as ligands in organometallic chemistry.
Cyclopentadienone can be generated by the photolysis or pyrolysis of various substances (e.g. 1,2-benzoquinone), and then isolated in an argon matrix at . It dimerizes readily upon thawing the matrix at .