A nitrobenzoxadiazole (NBD)‐based colorimetric chemosensor CNA (N,N,N‐trimethyl‐2‐(2‐(7‐nitrobenzo[c][1,2,5]oxadiazol‐4‐yl)hydrazineyl)‐2‐oxoethan‐1‐aminium chloride) was synthesized. Sensor CNA detected ClO− via color variation of yellow to colorless. The detection limit for ClO− by CNA was analyzed to be 0.36 μM. Moerover, the naked‐eye test strip was successfully applied for ClO− detection. The probing mechanism of CNA to ClO− was proposed to be a hydrolytic cleavage reaction of CNA by ClO− and demonstrated by ESI‐MS, FT‐IR, 1H NMR titration and DFT calculations.This article is protected by copyright. All rights reserved.