Diesel fuel , also called diesel 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 abbreviated DERV, standing for diesel-engined road vehicle, which carries a tax premium over equivalent fuel for non-road use. 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, 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.

The Lanzhou–Zhengzhou–Changsha product oil pipeline is a pipeline carrying diesel and other oil products from the northwest to the central regions of China. The 2,070-kilometre (1,290 mi) pipeline starts in Lanzhou in Gansu, and runs through Zhengzhou in Henan to Changsha in Hunan. In Zhengzhou, it is linked with Jinzhou–Zhengzhou product pipeline running from Jinzhou in Liaoning to Zhengzhou.The pipeline project was approved in 2007. The 1,188-kilometre (738 mi) long Lanzhou–Zhengzhou segment started operating on 30 March 2009. The Zhengzhou–Changsha segment is expected to be completed by the end of June 2010. The pipeline has an annual transportation capacity of 15 million tonnes. It is operated by PetroChina.