In cell biology a centriole is a cylindrical organelle composed mainly of a protein called tubulin. Centrioles are found in most eukaryotic cells, but are not present in conifers (Pinophyta), flowering plants (angiosperms) and most fungi, and are only present in the male gametes of charophytes, bryophytes, seedless vascular plants, cycads, and Ginkgo. A bound pair of centrioles, surrounded by a highly ordered mass of dense material, called the pericentriolar material (PCM), makes up a structure called a centrosome.

Centrioles are typically made up of nine sets of short microtubule triplets, arranged in a cylinder. Deviations from this structure include crabs and Drosophila melanogaster embryos, with nine doublets, and Caenorhabditis elegans sperm cells and early embryos, with nine singlets. Additional proteins include centrin, cenexin and tektin.

Centrioles are involved in the formation of cilia during interphase, as well as in organizing the aster and mitotic spindle during cell division.

The centrosome was discovered jointly by Walther Flemming in 1875 and Edouard Van Beneden in 1876. Edouard Van Beneden made the first observation of centrosomes as composed of two orthogonal centrioles in 1883. Theodor Boveri introduced the term "centrosome" in 1888 and the term "centriole" in 1895. The basal body was named by Theodor Wilhelm Engelmann in 1880. The pattern of centriole duplication was first worked out independently by Étienne de Harven and Joseph G. Gall c. 1950.

Centrioles are involved in the organization of the mitotic spindle and in the completion of cytokinesis. Centrioles were previously thought to be required for the formation of a mitotic spindle in animal cells. However, more recent experiments have demonstrated that cells whose centrioles have been removed via laser ablation can still progress through the G₁ stage of interphase before centrioles can be synthesized later in a de novo fashion. Additionally, mutant flies lacking centrioles develop normally, although the adult flies' cells lack flagella and cilia and as a result, they die shortly after birth. The centrioles can self replicate during cell division.

Centrioles are a key component of centrosomes, which are the cell's primary microtubule-organizing centers (MTOCs) and play a central role in organizing the cytoplasmic microtubule network. Through this role, centrioles contribute to nuclear positioning and the spatial arrangement of the cell.

Sperm centrioles are important for two functions: formation of the sperm flagellum and sperm motility, and development of the embryo after fertilization. In many organisms, the sperm contributes one or more centriole-like structures to the zygote that participate in the formation of the centrosome during early embryonic development. The number and structure of these sperm-derived centrioles vary across species. For example, in humans, the oocyte is acentriolar, and the sperm donates two structurally distinct centrioles: the proximal centriole (PC) and the distal centriole (DC), which contribute to formation of the first centrosome in the zygote. The centrosome formed from these sperm-derived centrioles organizes the microtubule network required for pronuclear migration and mitotic spindle assembly during early embryonic divisions.

In flagellates and ciliates, the position of the flagellum or cilium is determined by the mother centriole, which becomes the basal body. An inability of cells to use centrioles to make functional flagella and cilia has been linked to a number of genetic and developmental diseases. In particular, the inability of centrioles to properly migrate prior to ciliary assembly has recently been linked to Meckel–Gruber syndrome.