A high second harmonic generation response is demonstrated by a Pb ₆O ₅(NO ₃) ₂ lead oxynitrate whose identity was verified upon reinvestigation of the PbO-Pb(NO ₃) ₂ system. Its crystal structure exhibits a unique cationic [Pb ₆O ₅] ²⁺ framework hosting orientationally ordered NO ₃ ^- triangles in the channels. Easy preparation and high thermal stability (until ∼500 °C in air) suggest it to be a new promising NLO material.