In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or in the terminal position. Terminal alkenes are also known as α-olefins.

The International Union of Pure and Applied Chemistry (IUPAC) recommends using the name "alkene" only for acyclic hydrocarbons with just one double bond; alkadiene, alkatriene, etc., or polyene for acyclic hydrocarbons with two or more double bonds; cycloalkene, cycloalkadiene, etc. for cyclic ones; and "olefin" for the general class – cyclic or acyclic, with one or more double bonds.

Acyclic alkenes, with only one double bond and no other functional groups (also known as mono-enes) form a homologous series of hydrocarbons with the general formula C_nH_2n with n being 2 or more (which is two hydrogens less than the corresponding alkane). When n is four or more, isomers are possible, distinguished by the position and conformation of the double bond.

Alkenes are generally colorless non-polar compounds, somewhat similar to alkanes but more reactive. The first few members of the series are gases or liquids at room temperature. The simplest alkene, ethylene (C₂H₄) (or "ethene" in the IUPAC nomenclature) is the organic compound produced on the largest scale industrially.

Aromatic compounds are often drawn as cyclic alkenes, however their structure and properties are sufficiently distinct that they are not classified as alkenes or olefins. Hydrocarbons with two overlapping double bonds (C=C=C) are called allenes—the simplest such compound is itself called allene—and those with three or more overlapping bonds (C=C=C=C, C=C=C=C=C, etc.) are called cumulenes.