Red mud is a solid waste product of the Bayer process, the principal industrial means of refining bauxite in order to provide alumina as raw material for the electrolysis of aluminium by the Hall–Héroult process. A typical plant produces one to two times as much red mud as alumina. This ratio is dependent on the type of bauxite used in the refining process.

Red mud is composed of a mixture of solid and metallic oxide-bearing impurities, and presents one of the aluminium industry's most important disposal problems. The red colour is caused by the oxidised iron present, which can make up to 60% of the mass of the red mud. In addition to iron, the other dominant particles include silica, unleached residual aluminium, and titanium oxide.

Red mud cannot be disposed of easily. In most countries where red mud is produced, it is pumped into holding ponds. Red mud presents a problem as it takes up land area and can neither be built on nor farmed, even when dry. Due to the Bayer process the mud is highly basic with a pH ranging from 10 to 13. Several methods are used to lower the alkaline pH to an acceptable level to decrease the impact on the environment. Research is done to find a suitable way to utilize the mud for other applications, but drying the mud requires much energy (latent heat for water evaporation) and can represent high costs if fossil fuels have to be used in the drying process.

In October 2010, approximately one million cubic meters of red mud from an alumina plant near Kolontár in Hungary were accidentally released into the surrounding countryside in the Ajka alumina plant accident, killing nine people and contaminating a large area. All life in the Marcal river was said to have been "extinguished" by the red mud, and within days the mud had reached the Danube river.