Endosperm is a triploid tissue (containing three sets of chromosomes) found in the seeds of flowering plants. It provides nutrition to the developing embryo. It is mostly composed of starch, though it can also contain oils and protein.

Endosperm is formed when the two sperm inside a pollen grain reach the interior of an ovary. One sperm fertilizes the ovule, forming a fertilized egg, while the other sperm fertilizes the two polar bodies within the center of the ovary, creating endosperm.

Some plant seeds (such as peas) absorb the endosperm into their cotyledons, which then became the major source of nutrition during development. Most, however, keep the tissue as-is.

Cereal crops are grown for their palatable fruits (grains), which contain a great deal of endosperm. The endosperm is the part that is usually favored for eating, the other parts of the fruit being either removed or ignored. Endosperm thus has a vitally important role within the human diet, worldwide.