Mazut (Russian: Мазут, romanized: Mazut) is a low-quality heavy fuel oil, used in power plants and similar applications. In the United States and Western Europe, by using FCC or RFCC processes, mazut is blended or broken down, with the end product being diesel. Mazut may be used for heating houses in the former USSR and in countries of the Far East that do not have the facilities to blend or break it down into more conventional petro-chemicals. In the West, furnaces that burn mazut are commonly called "waste oil" heaters or "waste oil" furnaces.

There have been signs of mazut burning in Iran to compensate for the shortage of natural gas but it has caused environmental problems notably causing huge amounts of air pollution in big cities such as Tehran.

Mazut-100 is a fuel oil that is manufactured to GOST specifications, for example, GOST 10585-75 (not active) or GOST 10585-2013 (active as per December 2019). Mazut is almost exclusively manufactured in Russia, Kazakhstan, Azerbaijan, and Turkmenistan. This product is typically used for larger boilers in producing steam, since the energy value is high.

The most important factor when grading this fuel is the sulfur content, which can mostly be affected by the source feedstock. For shipment purposes, this product is considered a "dirty oil" product, and because viscosity drastically affects whether it is able to be pumped, shipping has unique requirements. Mazut is much like No. 6 fuel oil (Bunker C) and is part of the products left over after gasoline and lighter components are evaporated from the crude oil.