Fluorescence (FL) imaging in the second near-infrared window (NIR-II, 1000-1700 nm) has emerged as a promising bioimaging modality that enables noninvasive visualization of deep tissue with an unprecedented resolution. However, there is a paucity of studies on high-quality responsive NIR-II FL molecules. Herein we report a novel activated NIR-II FL molecule, 4,7-bis(5-(4-(diphenylamine)phenyl)-2-thiophene) [1,2,5]selenadiazolo[3,4-f]benzo[c][1,2,5]thiadiazole (SeTT), which exhibits fast and specific responsive capability to hypochlorous acid (HClO). To obtain the NIR-II ratiometric nanoprobe, SeTT was encapsulated on the surface of Er3+-doped down-conversion nanoparticles (DCNP), achieving the DCNP@SeTT nanoprobe. With a single 980 nm laser excitation, the ratiometric FL signal of SeTT at 1150 nm and DCNP at 1550 nm (I1150 nm/I1550 nm) was linearly correlated with the concentration of HClO with a detection limit of 0.4 μM. The ratiometric nanoprobe was successfully investigated for variations in HClO concentration in the tumor progression, visualization of anatomical structures of the peritoneal cavity in the mice model with inflammation, and quantitative detection of the HClO concentration in a rabbit model of osteoarthritis, achieving a fast response and high selectivity for the detection of HClO. The NIR-II-responsive nanoprobe can serve as a promising and effective tool for highly sensitive monitoring and imaging of HClO in living systems.