A simple and sensitive colorimetric assay for detecting organophosphorus pesticides (OPs) was developed based on 3,3',5,5'-tetramethylbenzidine (TMB)/hydrogen peroxide (H2O2)/dodecyl trimethylammonium bromide (DTAB)-tetramethyl zinc (4-pyridinyl) porphyrin (ZnTPyP). In this system, based on the peroxidase-like activity of DTAB-ZnTPyP, H2O2 decomposes to produce hydroxyl radicals, which oxidize TMB, resulting in blue oxidation products. The OPs (trichlorfon, dichlorvos, and thimet) were first combined with DTAB-ZnTPyP through electrostatic interactions. The OPs caused a decrease in the peroxidase-like activity of DTAB-ZnTPyP due to spatial site blocking. At the same time, π-interactions occurred between them, and these interactions also inhibited the oxidation of TMB (652 nm), thus making the detection of OPs possible. The limits of detection for trichlorfon, dichlorvos, and thimet were 0.25, 1.02, and 0.66 μg/L, respectively, and the corresponding linear ranges were 1–35, 5–45, and 1–40 μg/L, respectively. Moreover, the assay was successfully used to determine OPs in cabbage, apple, soil, and traditional Chinese medicine samples (the recovery ratios were 91.8–109.8%), showing a great promising potential for detecting OPs also in other complex samples. Highlights: A novel assay based on the peroxidase-like DTAB-ZnTPyP was developed to detect OPs. DTAB-ZnTPyP interacts more strongly with OPs than other nanoporphyrins. This colorimetric assay exhibits a low detection limit of 0.25 μg/L for OPs. The assay was applied to a real sample with good results. [ABSTRACT FROM AUTHOR]