A marrow is a fruit used as a vegetable, the mature fruit of certain Cucurbita pepo cultivars. The immature fruit of the same or similar cultivars is called courgette (in Britain, Iran, Ireland, France, the Netherlands, Singapore, Malaysia and New Zealand) or zucchini (in North America, Japan, Australia, the Czech Republic, Italy, Germany and Austria). Like courgettes, marrows are oblong, green squash, but marrows have a firm rind and a neutral flavour ("overgrown when picked and insipid when cooked"), making them useful as edible casings for mincemeat and other stuffings. They can be stored for several weeks after harvest (like pumpkins and other winter squash), to be processed for food when required. They are a vegetable used in Great Britain and areas with significant British influence, though their popularity is waning in favor of immature summer squash like courgette.

Giant marrows are grown competitively in the United Kingdom, where the term "marrow" is often restricted to the striped, thick-skinned cultivar.

In a culinary context, marrows are treated as a vegetable; usually cooked and presented as a savory dish or accompaniment. Botanically, marrows are fruit, a type of botanical berry, being the swollen ovary of the marrow flower. Marrows, like all squash, have their ancestry in the Americas.