Diesel fuel, also known as diesel oil, is a liquid specifically designed for use in a diesel engine, which is a type of internal combustion engine where fuel ignition occurs without a spark due to compression of the inlet air followed by fuel injection. Therefore, diesel fuel requires good compression ignition characteristics.

Diesel fuel originated from experiments conducted by German scientist and inventor Rudolf Diesel for his compression-ignition engine, which he invented around 1892. Originally, Diesel did not specify a particular type of fuel, instead claiming that the operating principle of his rational heat motor would function with any fuel in any state of matter. The first diesel engine prototype and the first functional Diesel engine were designed exclusively for liquid fuels.

Diesel fuel is produced from various sources, with the most common being petroleum. Other sources include biomass, animal fat, biogas, natural gas, and coal liquefaction.