Nitrous oxide, commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula N
2O. At room temperature, it is a colourless non-flammable gas, with a slight metallic scent and taste. At elevated temperatures, nitrous oxide is a powerful oxidiser similar to molecular oxygen.

Nitrous oxide has significant medical uses, especially in surgery and dentistry, for its anaesthetic and pain reducing effects. Its colloquial name "laughing gas", coined by Humphry Davy, is due to the euphoric effects upon inhaling it, a property that has led to its recreational use as a dissociative anaesthetic. It is on the World Health Organisation's List of Essential Medicines, the safest and most effective medicines needed in a health system. It is also used as an oxidiser in rocket propellants, and in motor racing to increase the power output of engines.

Nitrous oxide's atmospheric concentration reached 333 parts per billion (ppb) in 2020, increasing at a rate of about 1 ppb annually. It is a major scavenger of stratospheric ozone, with an impact comparable to that of CFCs. Global accounting of N
2O sources and sinks over the decade ending 2016 indicates that about 40% of the average 17 TgN/yr (Teragrams of Nitrogen per year) of emissions originated from human activity, and shows that emissions growth chiefly came from expanding agriculture and industry sources within emerging economies. Being the third most important long-lived greenhouse gas, nitrous oxide also substantially contributes to global warming.