Diacetylene (also known as butadiyne) is the organic compound with the formula C4H2.

It is first in the series of polyynes, which are of theoretical but not of practical interest.

Diacetylene has been identified in the atmosphere of Titan and in the protoplanetary nebula CRL 618 by its characteristic vibrational spectrum.

This radical can in turn attack the triple bond in acetylene and react efficiently even at low temperatures.

This compound may be made by the dehydrohalogenation of 1,4-dichloro-2-butyne by potassium hydroxide (in alcoholic medium) at ~70°C:[1] The bis(trimethylsilyl)-protected derivative may be prepared by the Hay coupling of (trimethylsilyl)acetylene:[2]