Hyponitrous acid is a chemical compound with formula H
₂N
₂O
₂ or HON=NOH. It is an isomer of nitramide, H₂N−NO₂; and a formal dimer of azanone, HNO.

Hyponitrous acid forms two series of salts, the hyponitrites containing the [ON=NO]²⁻ anion, and the "acid hyponitrites" containing the [HON=NO]⁻ anion.^[1]

There are two possible structures of hyponitrous acid, trans and cis. trans-Hyponitrous acid forms white crystals that are explosive when dry. In aqueous solution, it is a weak acid (pK_a1 = 7.21, pK_a2 = 11.54), and decomposes to nitrous oxide and water with a half life of 16 days at 25 °C at pH 1–3:

${\displaystyle {\ce {H2N2O2 -> H2O + N2O}}}$

Since this reaction is not reversible, N
₂O should not be considered as the anhydride of H
₂N
₂O
₂.^[1]

The cis acid is not known,^[1] but its sodium salt can be obtained.^[2]

Hyponitrous acid (trans) can be prepared from silver(I) hyponitrite and anhydrous HCl in ether:

${\displaystyle {\ce {Ag2N2O2 + 2 HCl -> H2N2O2 + 2 AgCl}}}$

Spectroscopic data indicate a trans configuration for the resulting acid.^[2]
It can also be synthesized from hydroxylamine and nitrous acid:

${\displaystyle {\ce {NH2OH + HNO2 -> H2N2O2 + H2O}}}$

In enzymology, a hyponitrite reductase is an enzyme that catalyzes the chemical reaction^[3]

${\displaystyle {\ce {H2N2O2 + 2 NADH + 2 H+ <-> 2 NH2OH + 2 NAD+}}}$