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Boronate-based fluorescent probes are widely used for the imaging of hydrogen peroxide (H 2 O 2). However, their selectivity might be subjected to the interference of other reactive oxygen species, especially peroxynitrite (ONOO-), due to the reaction of boronates with ONOO- being several orders of magnitude faster than with H 2 O 2. This work highlights a doubly activated electrophilicity-increasing strategy to develop a boronate-based fluorescent probe THMP for selective imaging of H 2 O 2 over ONOO-, where a boronate-modified pyridiniumylacrylonitrile is grafted on the 2-(2′-hydroxy-3′-methoxyphenyl) benzothiazole scaffold. Specifically, the boronate oxidation of THMP by H 2 O 2 leads to the release of the free fluorophore THMP-N, triggering a ratiometric fluorescence response from red to green. The presence of a doubly activated electrophilic site on the carbon-carbon double bond of THMP, by both the strong electron-withdrawing cyano group and the pyridinium moiety, allows selective oxidative cleavage of the double bond by ONOO- to an aldehyde HMBT-CHO, thereby excluding the interference of ONOO- in monitoring H 2 O 2. With the aid of the probe, we successfully visualized increased levels of H 2 O 2 during ferroptosis of HepG2 cells, and burst of H 2 O 2 in brains of live mice during cerebral ischemia reperfusion injury. [Display omitted] • Highlighting a doubly activated electrophilicity-increasing strategy. • Developing a boronate-based fluorescent probe THMP based on the strategy. • Selective imaging of H 2 O 2 over ONOO- by THMP. • Detailed mechanistic investigation. • Visualizing H 2 O 2 in brains of live mice during cerebral ischemia reperfusion. [ABSTRACT FROM AUTHOR]