1-Butene (IUPAC name: But-1-ene, also known as 1-butylene) is the organic compound with the formula CH₃CH₂CH=CH₂. It is a colorless gas, but easily condensed to give a colorless liquid. It is classified as a linear alpha-olefin (terminal alkene). It is one of the isomers of butene (butylene). It is a precursor to diverse products.

Polymerization of 1-butene gives polybutylene, which is used to make piping for domestic plumbing. Another application is as a comonomer in the production of certain kinds of polyethylene, such as linear low-density polyethylene (LLDPE). It has also been used as a precursor to polypropylene resins, butylene oxide, and butanone.

1-Butene is produced by separation from crude C₄ refinery streams and by ethylene dimerization. The former affords a mixture of 1-and 2-butenes, while the latter affords only the terminal alkene. It is distilled to give a very high purity product. An estimated 12 billion kilograms were produced in 2011.