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Abstract We present the synthesis, properties, and imaging applications of a new class of diazaborine‐based probes (Peroxynitrite Probe‐1,PNP‐1) for selective peroxynitrite (ONOO⁻) imaging in live cells.PNP‐1features a diazaborine‐based reaction motif that provides excellent discrimination between H₂O₂and ONOO⁻, solving a persistent challenge of organoboron‐based fluorescent probes for oxidative metabolite imaging. We demonstrate the utility ofPNP‐1to detect endogenously produced ONOO⁻in live RAW 264.7 macrophages by fluorescence microscopy, with probe selectivity confirmed with inhibition of NADPH oxidases and nitric oxide synthase, the requisite enzymatic machinery for ONOO⁻production. Co‐localization studies unexpectedly reveal preferential mitochondrial localization, which we show is dependent on the naphthalimide scaffold. Taken together, our results show that diazaborines are a novel motif for selective ONOO⁻reactivity, positioning them for incorporation into other ONOO⁻‐specific chemical biology tools.