Ferrosilicon, or ferrosilicium, is a ferroalloy, an alloy of iron and silicon with between 15% and 90% silicon. It contains a high proportion of iron silicides. Its melting point is about 1200°C to 1250°C with a boiling point of 2355°C. It also contains about 1% to 2% of calcium and aluminium.

In steel industry, ferrosilicon is used as a source of silicon to deoxidize steel and other ferrous alloys. This prevents the loss of carbon from the molten steel, ferromanganese, spiegeleisen and silicides of calcium.

Ferrosilicon is also used for manufacture of silicon, corrosion-resistant and high-temperature resistant ferrous silicon alloys, and silicon steel for electromotors and transformer cores. In the manufacture of cast iron, ferrosilicon is used for inoculation of the iron to accelerate graphitization. In arc welding, ferrosilicon can be found in some electrode coatings.

Ferrosilicon is also used in the Pidgeon process to make magnesium from dolomite.

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