Diesel fuel ( /ˈdiːzəl/) in general is any liquid fuel used in diesel engines. The most common is a specific fractional distillate of petroleum fuel oil, but alternatives that are not derived from petroleum, such as biodiesel, biomass to liquid (BTL) or gas to liquid (GTL) diesel, are increasingly being developed and adopted. To distinguish these types, petroleum-derived diesel is increasingly called petrodiesel.Ultra-low sulfur diesel (ULSD) is a standard for defining diesel fuel with substantially lowered sulfur contents. As of 2007, almost all diesel fuel available in the United States of America, Canada and Europe is the ULSD type.

In the UK, diesel fuel for on-road use is commonly abbreviated DERV, standing for Diesel Engined Road Vehicle, which carries a tax premium over equivalent fuel for non-road use (see Taxation).

The word "diesel" is derived from the family name of German inventor Rudolf Diesel who in 1892 invented the diesel engine.

Diesel engines are a type of internal combustion engine. Rudolf Diesel originally designed the diesel engine to use coal dust as a fuel. He also experimented with various oils, including some vegetable oils, such as peanut oil, which was used to power the engines which he exhibited at the 1900 Paris Exposition and the 1911 World's Fair in Paris.