Nitroglycerin (NG) (alternative spelling nitroglycerine), also known as trinitroglycerol (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless or pale yellow, oily, explosive liquid most commonly produced by nitrating glycerol with white fuming nitric acid under conditions appropriate to the formation of the nitric acid ester. Chemically, the substance is a nitrate ester rather than a nitro compound, but the traditional name is retained. Discovered in 1846 by Ascanio Sobrero, nitroglycerin has been used as an active ingredient in the manufacture of explosives, namely dynamite, and as such it is employed in the construction, demolition, and mining industries. It is combined with nitrocellulose to form double-based smokeless powder, used as a propellant in artillery and firearms since the 1880s.

As is the case for many other explosives, nitroglycerin becomes more and more prone to exploding (i.e., spontaneous decomposition) as the temperature is increased. Upon exposure to heat above 218 °C at sea-level atmospheric pressure, nitroglycerin becomes extremely unstable and tends to explode. When placed in vacuum, it has an autoignition temperature of 270 °C instead. With a melting point of 12.8 °C, the chemical is almost always encountered as a thick and viscous fluid, changing to a crystalline solid when frozen. Although the pure compound itself is colorless, in practice the presence of nitric oxide impurities left over during production tends to give it a slight yellowish tint.

Due to its high boiling point and consequently low vapor pressure (0.00026 mmHg at 20 °C), pure nitroglycerin has practically no odor at room temperature, with a sweet and burning taste when ingested. Unintentional detonation may ensue when dropped, shaken, lit on fire, rapidly heated, exposed to sunlight and ozone, subjected to sparks and electrical discharges, or roughly handled. Its sensitivity to exploding is responsible for numerous devastating industrial accidents throughout its history. The chemical's characteristic reactivity may be reduced through the addition of desensitizing agents, which makes it less likely to explode. Clay (diatomaceous earth) is an example of such an agent, forming dynamite, a much more easily handled composition. The addition of other desensitizing agents gives birth to the various formulations of dynamite.

Nitroglycerin as a medication has been used since 1878 as a potent vasodilator (causing dilation of the vascular system) to treat heart conditions, such as angina pectoris and chronic heart failure. Though it was previously known that these beneficial effects are due to nitroglycerin being converted to nitric oxide, a potent vasodilator, the enzyme for this conversion was only discovered to be mitochondrial aldehyde dehydrogenase (ALDH2) in 2002. Nitroglycerin is available in sublingual tablets, sprays, ointments, and patches.

Nitroglycerin was the first practical explosive produced that was stronger than black powder. It was synthesized by the Italian chemist Ascanio Sobrero in 1846, working under Théophile-Jules Pelouze at the University of Turin. Sobrero initially called his discovery "pyroglycerin" and warned vigorously against its use as an explosive.

Nitroglycerin was adopted as a commercially useful explosive by Alfred Nobel, who experimented with safer ways to handle the dangerous compound after his younger brother, Emil Oskar Nobel, and several factory workers were killed in an explosion at the Nobels' armaments factory in 1864 in Heleneborg, Sweden.

One year later, Nobel founded Alfred Nobel and Company in Germany and built an isolated factory in the Krümmel hills of Geesthacht near Hamburg. This business exported a liquid combination of nitroglycerin and gunpowder called "Blasting Oil", but this was extremely unstable and difficult to handle, as evidenced in numerous catastrophes. The buildings of the Krümmel factory were destroyed twice.

In April 1866, several crates of nitroglycerin were shipped to California, three of which were destined for the Central Pacific Railroad, which planned to experiment with it as a blasting explosive to expedite the construction of the 1,659-foot-long (506 m) Summit Tunnel through the Sierra Nevada Mountains. One of the remaining crates exploded, destroying a Wells Fargo company office in San Francisco and killing 15 people. This led to a complete ban on the transportation of liquid nitroglycerin in California. The on-site manufacture of nitroglycerin was thus required for the remaining hard-rock drilling and blasting required for the completion of the First transcontinental railroad in North America.