A reversible and mitochondria targetable fluorescent probe (Nap-Se) bearing 1,8-naphthalimide and a selenomorpholine fragment was designed and synthesized. The probe showed a highly selective, sensitive and fast response (<2 s) toward hypochlorous acid/hypochlorite in phosphate buffer saline solution. Cell imaging studies confirmed that probe Nap-Se was specifically localized in mitochondria of living cells and could be used to monitor the levels of endogenous and exogenous HClO/ClO⁻ in RAW264.7 cells.