Pig iron is raw iron, the immediate product of smelting iron ore with coke and limestone in a blast furnace. Pig iron has a very high carbon content, typically 4-5%, which makes it very brittle and not very useful directly as a material.

Pig iron is typically poured directly out of the bottom of the blast furnace into pots to form ingots. The ingots are then used to produce wrought iron or steel, typically with a bessemer converter or basic oxygen furnace, by burning off the excess carbon in a controlled fashion.

Cast iron is made by remelting pig iron, often along with substantial quantities of scrap iron, and taking various steps to remove undesirable contaminants and adjust the carbon content.

The name is derived from the time when the iron ran into molds in sand beds fed from a common runner. The row of molds was said to resemble a litter of suckling pigs, hence the individual ingots were referred to as pigs and the runner was called the sow.

The Chinese were making pig iron by the later Zhou Dynasty (1122 BC - 256 BC). In Europe, the process did not become common until the 14th century.