线粒体或粒线体（mitochondrion），是一种存在于大多数真核细胞中的由两层膜包被的细胞器，直径在0.5到10微米左右。除了溶组织内阿米巴、篮氏贾第鞭毛虫以及几种微孢子虫外，大多数真核细胞或多或少都拥有线粒体，但它们各自拥有的线粒体在大小、数量及外观等方面上都有所不同。这种细胞器拥有自身的遗传物质和遗传体系，但因其基因组大小有限，所以线粒体是一种半自主细胞器。线粒体是细胞内氧化磷酸化和合成三磷酸腺苷（ATP）的主要场所，为细胞的活动提供了化学能量，所以有“细胞的发电站”（the powerhouse of the cell）之称。除了为细胞供能外，线粒体还参与诸如细胞分化、细胞信息传递和细胞凋亡等过程，并拥有调控细胞生长和细胞周期的能力。美国加州大学的一项最新研究，包括确诊患有严重自闭症的十个孩子，结果表明自闭症可能与线粒体缺陷密切相关。

The mitochondrion (plural mitochondria) is a double membrane-bound organelle found in most eukaryotic cells. The word mitochondrion comes from the Greek μίτος, mitos, i.e. "thread", and χονδρίον—, chondrion, i.e. "granule" or "grain-like".

Mitochondria are commonly between 0.75 and 3μm in diameter but vary considerably in size and structure. Unless specifically stained, they are not visible. Mitochondria have been described as "the powerhouse of the cell" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. In addition to supplying cellular energy, mitochondria are involved in other tasks, such as signaling, cellular differentiation, and cell death, as well as maintaining control of the cell cycle and cell growth. Mitochondria have been implicated in several human diseases, including mitochondrial disorders, cardiac dysfunction, and heart failure. In 2014, a study including ten children diagnosed with severe autism suggests that autism may be correlated with mitochondrial defects.