AbstractHypochlorous acid (HClO/ClO−) is one of the important reactive oxygen species (ROS). It acts as a second signaling molecule within and between cells and is an indispensable active molecule in living organisms to regulate physiological and pathological processes. In this article, two fluorescent probes (PTF and PTA) for highly selective fluorescent recognition of ClO− were successfully synthesized based on the ICT mechanism by condensing phenothiazines with two hydrazides via the hydrazide structure (). PTF can identify different concentrations of ClO− in two steps. Due to its ClO− two site recognition, the probe exhibited good selectivity (specific recognition of ClO− over a wide concentration range), a fast time response (rapid recognition in seconds), a sufficiently low detection limit (3.6 and 11.0 nM), and large Stokes shifts (180 and 145 nm). Furthermore, the recognition of ClO− by contrasting probes with different substituents exhibited different fluorescence changes of ratiometric type and turn‐off. PTF successfully achieves the detection of exogenous and endogenous ClO− in aqueous solution and living cells.