Winter squash is an annual fruit representing several squash species within the genus Cucurbita. Late-growing, less symmetrical, odd-shaped, rough or warty varieties, small to medium in size, but with long-keeping qualities and hard rinds, are usually called winter squash. They differ from summer squash in that they are harvested and eaten in the mature stage when their seeds within have matured fully and their skin has hardened into a tough rind. At this stage, most varieties of this vegetable can be stored for use during the winter. Winter squash is generally cooked before being eaten, and the skin or rind is not usually eaten as it is with summer squash.

Cultivars of winter squash that are round and orange are called pumpkins. In New Zealand and Australian English, the term "pumpkin" generally refers to the broader category called "winter squash".

Although winter squashes are grown in many regions, they are relatively economically unimportant, with few exceptions. They are grown extensively in tropical America, Japan, Northern Italy, and certain areas of the United States. The calabazas of the West Indies and the forms grown by the people of Mexico and Central America are not uniform, pure varieties but extremely variable in size, shape, and color. Since these species are normally cross-pollinated, it is now difficult to keep a variety pure.