烯烃（拼音：xī tīng，注音：ㄒㄧ ㄊㄧㄥ，音同「希 听」；英语：Alkene）是指含有C=C键（碳－碳双键）的碳氢化合物。属于不饱和烃，分为链烯烃与环烯烃。按含双键的多少分别称单烯烃、二烯烃等。只拥有一个双键的简单烯烃组成了一个由通式C_nH_2n（其中 n ≥ 2）代表的同系物。

最简单的烯烃是乙烯（C₂H₄），国际纯化学和应用化学联合会命名为ethene。乙烯是最大规模工业上生产的有机化合物。

“烯”字是新造字，左边的火取自“碳”字，代表可以燃烧；右边的希取自“稀”字，表示氢原子和化合价稀少之意，意味着烯是烷（完整）烃的一类不饱和衍生物。

In organic chemistry, an alkene is an unsaturated hydrocarbon that contains at least one carbon–carbon double bond. The words alkene, olefin, and olefine are used often interchangeably (see nomenclature section below). Acyclic alkenes, with only one double bond and no other functional groups, known as mono-enes, form a homologous series of hydrocarbons with the general formula C_nH_2n. Alkenes have two hydrogen atoms less than the corresponding alkane (with the same number of carbon atoms). The simplest alkene, ethylene (C₂H₄), with the International Union of Pure and Applied Chemistry (IUPAC) name ethene, is the organic compound produced on the largest scale industrially. Aromatic compounds are often drawn as cyclic alkenes, but their structure and properties are different and they are not considered to be alkenes.