Hydrazine is an inorganic compound with the chemical formula N
2H
4. It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour.

Hydrazine is highly toxic unless handled in solution as, for example, hydrazine hydrate (NH
2NH
2 · xH
2O). As of 2015, the world hydrazine hydrate market amounted to $350 million. Hydrazine is mainly used as a foaming agent in preparing polymer foams, but applications also include its uses as a precursor to polymerization catalysts, pharmaceuticals, and agrochemicals, as well as a long-term storable propellant for in-space spacecraft propulsion.

About two million tons of hydrazine hydrate were used in foam blowing agents in 2015. Additionally, hydrazine is used in various rocket fuels and to prepare the gas precursors used in air bags. Hydrazine is used within both nuclear and conventional electrical power plant steam cycles as an oxygen scavenger to control concentrations of dissolved oxygen in an effort to reduce corrosion.

Hydrazines refer to a class of organic substances derived by replacing one or more hydrogen atoms in hydrazine by an organic group.