A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. Blast refers to the combustion air being supplied above atmospheric pressure.

In a blast furnace, fuel (coke), ores, and flux (limestone) are continuously supplied through the top of the furnace, while a hot blast of (sometimes oxygen-enriched) air is blown into the lower section of the furnace through a series of pipes called tuyeres, so that the chemical reactions take place throughout the furnace as the material falls downward. The end products are usually molten metal and slag phases tapped from the bottom, and flue gases exiting from the top. The downward flow of the ore along with the flux in contact with an upflow of hot, carbon monoxide-rich combustion gases is a countercurrent exchange and chemical reaction process.

In contrast, air furnaces (such as reverberatory furnaces) are naturally aspirated, usually by the convection of hot gases in a chimney flue. According to this broad definition, bloomeries for iron, blowing houses for tin, and smelt mills for lead would be classified as blast furnaces. However, the term has usually been limited to those used for smelting iron ore to produce pig iron, an intermediate material used in the production of commercial iron and steel, and the shaft furnaces used in combination with sinter plants in base metals smelting.

Blast furnaces are estimated to have been responsible for over 4% of global greenhouse gas emissions between 1900 and 2015, and are difficult to decarbonize.

Blast furnaces operate on the principle of chemical reduction whereby carbon monoxide converts iron oxides to elemental iron.

Blast furnaces differ from bloomeries and reverberatory furnaces in that in a blast furnace, flue gas is in direct contact with the ore and iron, allowing carbon monoxide to diffuse into the ore and reduce the iron oxide. The blast furnace operates as a countercurrent exchange process whereas a bloomery does not. Another difference is that bloomeries operate as a batch process whereas blast furnaces operate continuously for long periods. Continuous operation is also preferred because blast furnaces are difficult to start and stop. Also, the carbon in pig iron lowers the melting point below that of steel or pure iron; in contrast, iron does not melt in a bloomery.

Silica has to be removed from the pig iron. It reacts with calcium oxide (burned limestone) and forms silicates, which float to the surface of the molten pig iron as slag. Historically, iron was produced with charcoal to prevent sulfur contamination.

In a blast furnace, a downward-moving column of ore, flux, coke (or charcoal) and their reaction products must be sufficiently porous for the flue gas to pass through, upwards. To ensure this permeability the particle size of the coke or charcoal is of great relevance. Therefore, the coke must be strong enough so it will not be crushed by the weight of the material above it. Besides the physical strength of its particles, the coke must also be low in sulfur, phosphorus, and ash.