Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide (NTO), and occasionally (usually among ex-USSR/Russian rocket engineers) as amyl, is the chemical compound N₂O₄. It is a useful reagent in chemical synthesis. It forms an equilibrium mixture with nitrogen dioxide. Its molar mass is 92.011 g/mol.

Dinitrogen tetroxide is a powerful oxidizer that is hypergolic (spontaneously reacts) upon contact with various forms of hydrazine, which has made the pair a common bipropellant for rockets.

Dinitrogen tetroxide could be regarded as two nitro groups (-NO₂) bonded together. It forms an equilibrium mixture with nitrogen dioxide. The molecule is planar with an N-N bond distance of 1.78 Å and N-O distances of 1.19 Å. The N-N distance corresponds to a weak bond, since it is significantly longer than the average N-N single bond length of 1.45 Å. This exceptionally weak σ bond (amounting to overlapping of the sp² hybrid orbitals of the two NO₂ units) results from the simultaneous delocalization of the bonding electron pair across the whole N₂O₄ molecule, and the considerable electrostatic repulsion of the doubly occupied molecular orbitals of each NO₂ unit.

Unlike NO₂, N₂O₄ is diamagnetic since it has no unpaired electrons. The liquid is also colorless but can appear as a brownish yellow liquid due to the presence of NO₂ according to the following equilibrium:

Higher temperatures push the equilibrium towards nitrogen dioxide. Inevitably, some dinitrogen tetroxide is a component of smog containing nitrogen dioxide.

Solid N₂O₄ is white, and melts at −11.2 °C.

Nitrogen tetroxide is made by the catalytic oxidation of ammonia (the Ostwald process): steam is used as a diluent to reduce the combustion temperature. In the first step, the ammonia is oxidized into nitric oxide:

Most of the water is condensed out, and the gases are further cooled; the nitric oxide that was produced is oxidized to nitrogen dioxide, which is then dimerized into nitrogen tetroxide: