Acorn or acorn nut is the nut of oak and its close relatives (Quercus and Quercus). It usually consists of one seed (sometimes two), wrapped in a hard leather shell and propagated in a cup-shaped sucker. Acorn is 1-6cm long and 0.8cm-4cm wide. Acorns take 6 to 24 months (depending on species) to mature; for more information on oak classification, please refer to the oak species list, and acorn morphology and phenology are important factors.

Acorn plays an important role in forest ecosystem when oak is dominant or abundant. Acorn crops can vary greatly in size, causing great abundance or pressure on many animals that rely on acorns and their natural enemies. Acorns, along with other nuts, are called masts.

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