Residual fuel is any petroleum-based fuel which contains the undistilled residue from atmospheric or vacuum distillation of crude oil. It is also known as Heavy Fuel Oil, Bunker Fuel Oil or No. 6 Fuel Oil.

As a byproduct of refining gasoline and diesel fuel, it is the cheapest liquid fuel available. However the emission of carbon dioxide, sulfur and nitrogen oxides during combustion is much greater. Hence it pollutes more than distillate or gaseous fuels (but less than solid fuels such as coal).

In the mid twentieth century, heavy fuel oil was commonly used to power industrial boilers, railroad locomotives and steamships. It was gradually used to generate electricity, supplying 16.8 percent of electricity generation in the United States by 1973. Reliance on heavy fuel oil was reduced as oil prices rose in the 1970s and by 1983 it accounted for just 6.2 percent of the electricity generated.