Mazut is a heavy petroleum fuel consisting primarily of heavy hydrocarbons, long-chain carbon compounds, and varying amounts of sulfur. It is obtained from the refining of crude oil and the heavier residual fractions generated during refinery processes.

Mazut is combustible and typically has a flash point in the range of approximately 60–100°C, depending on the product type and grade.

It is primarily used in power plants, large-scale industrial facilities, and industrial furnaces and boilers.