Diesel fuel , also called diesel oil or historically heavy oil, is any liquid fuel specifically designed for use in a diesel engine, a type of internal combustion engine in which fuel ignition takes place without a spark as a result of compression of the inlet air and then injection of fuel. Therefore, diesel fuel needs good compression ignition characteristics.

The most common type of diesel fuel 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 sometimes called petrodiesel in some academic circles.

In many countries, diesel fuel is standardised. For example, in the European Union, the standard for diesel fuel is EN 590. Diesel fuel has many colloquial names; most commonly, it is simply referred to as diesel. In the United Kingdom, diesel fuel for on-road use is commonly called diesel or sometimes white diesel if required to differentiate it from a tax-advantaged agricultural-only product containing an identifying coloured dye known as red diesel. The official term for white diesel is DERV, standing for diesel-engine road vehicle. In Australia, diesel fuel is also known as distillate (not to be confused with "distillate" in an older sense referring to a different motor fuel), and in Indonesia (as well in Israel), it is known as Solar, a trademarked name from the country's national petroleum company Pertamina. The term gas oil (French: gazole) is sometimes also used to refer to diesel fuel.

Ultra-low-sulfur diesel (ULSD) is a diesel fuel with substantially lowered sulfur contents. As of 2016, almost all of the petroleum-based diesel fuel available in the United Kingdom, mainland Europe, and North America is of a ULSD type.

Before diesel fuel had been standardised, the majority of diesel engines typically ran on cheap fuel oils. These fuel oils are still used in watercraft diesel engines. Despite being specifically designed for diesel engines, diesel fuel can also be used as fuel for several non-diesel engines, for example the Akroyd engine, the Stirling engine, or boilers for steam engines.