The pure rotational spectrum of cis–cis peroxynitrous acid, HOONO, has been observed. Over 220 transitions, sampling states up to J'=67 and K'a=31, have been fitted with an rms uncertainty of 48.4 kHz. The experimentally determined rotational constants agree well with ab initio values for the cis–cis conformer, a five-membered ring formed by intramolecular hydrogen bonding. The small, positive inertial defect Δ=0.075667(60) amu Å2 and lack of any observable torsional splittings in the spectrum indicate that cis–cis HOONO exists in a well-defined planar structure at room temperature.