In biology, a syncytium is a large region of cytoplasm that contains many nuclei. A syncytium can form in two ways. Many insects have early embryos in which cell division is incomplete. Nuclei reproduce in the absence of cytokinesis inside the large cytoplasmic space of the zygote. Large skeletal muscle fibers form by the fusion of thousands of myocytes.

The early embryo syncytium of invertebrates such as Drosophila is important for syncitial specification of cell differentiation. The egg cell cytoplasm contains gradients of mRNA molecules such as those that encode the protein Bicoid and the protein Nanos. Bicoid protein becomes concentrated at the anterior end of the early embryo while Nanos protein is concentrated at the posterior end. As the nuclei of the early embryo rapidly divide they distribute through the cytoplasm where they are exposed to the different amounts of Bicoid and Nanos. Those nuclei with more Bicoid will differentiate into cells of body regions such as the thorax. Nuclei exposed to more Nanos will differentiate into cells of body regions such as the abdomen.

The syncytium of skeletal muscle is important because it allows rapid coordinated contraction of muscles along their entire length. Action potentials propagate along the surface of the muscle fiber from the point of synaptic contact with a motoneuron. Another important vertebrate syncytium is in the placenta of placental mammals. Embryo-derived cells that form the interface with the maternal blood stream fuse together to form a multi-nucleated barrier. This is probably important in order to limit the exchange of migratory cells between the developing embryo and the body of the mother. Some blood cells are specialized to be able to insert themselves between adjacent epithelial cells. The syncitial epithelium of the placenta does not provide such an access path from the maternal circulation into the embryo.