We have observed a cation with atomic composition [N3O](+) via time-of-flight mass spectroscopy. This cation was observed in the reaction products of detonated liquid nitromethane. The same 58 amu species was observed when protonated, deuterated, and C-13-labeled nitromethane were detonated; therefore, the only possible atomic composition of the cation is [N3O](+). The existence of a metastable form of [N3O](+) has current interest since this cation is isoelectronic with electrically neutral N-4. There is interest in the metastability of N-4 structures because such species would have very high energy content and would be very powerful explosives and propellants. We present evidence that the [N3O](+) structure we observe is chemically bound. The results of ab initio quantum chemical calculations at various levels of theory are presented for three [N3O](+) structures. These calculations indicate that two of the structures are metastable and have high energy content; calculated vibrational frequencies are presented for these two forms. A lower bound on the lifetime of the observed cation is 0.52 mu s. Our results strengthen the idea that very high energy metastable forms of N-4 can exist.