纤毛（拉丁语：cilium，复数为cilia）是真核生物细胞的胞器（细胞器），是一种尾状的突出物，伸向细胞外大约5到10微米。支撑纤毛的亚显微结构是微管，微管连接在基体之上。 动物细胞利用纤毛拨动附近或移动自身，分布在呼吸系统、输卵管，像是气管壁上的细胞就具有纤毛，可以排除肺部尘埃。

纤毛可分为两种型态，一种称为运动纤毛（motile cilia），能够长久地向同一方向运动。另一种则是非运动纤毛（non-motile cilia），一般用作感应胞器。 纤毛与鞭毛合称为波动足（un****podia）。 纤毛会朝着向外的方向摆动，使异物随着纤毛的摆动而排出。

A cilium (Latin for eyelash; the plural is cilia) is an organelle found in eukaryotic cells. Cilia are thick protuberances that project from the much larger cell body.

There are two types of cilia: motile cilia and nonmotile, or primary cilia, which typically serve as sensory organelles. In eukaryotes, motile cilia and flagella together make up a group of organelles known as ****. Eukaryotic cilia are structurally identical to eukaryotic flagella, although distinctions are sometimes made according to function and/or length. Biologists have various ideas about how the various flagella may have evolved.