<p>Developing highly sensitive, selective, and rapid detection methods for methyl parathion (MP)&#xa0;-&#xa0;a highly toxic organophosphorus pesticide&#xa0;-&#xa0;is critically important. Conventional MP detection methods suffer from complexity, high instrument cost, and slow analysis. To overcome these limitations, this study presents a photoelectrochemical transistor (OPECT) sensor based on a ZnIn₂S₄/TiO₂ nanorod array (NRA) Z-scheme heterojunction for ultra-sensitive and selective MP detection. Under illumination, the heterojunction efficiently separates photogenerated charges and produces a photocurrent that acts as a gate potential. This drives cation migration from the electrolyte to the poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS) channel interface, modulating the channel current. MP’s phosphate ester groups selectively bind Zn²⁺ ions, hindering electron transfer between the donor and gate material. This changes the effective gate voltage and causes a significant shift in channel current, enabling sensitive and selective detection. The OPECT sensor shows a strong linear response from 10 ng/mL to 10&#xa0;fg/mL, with a detection limit of 1.69&#xa0;fg/mL, surpassing most existing methods. Meanwhile, the OPECT detection of MP in water and apple samples were also conducted, with recoveries of 89.27%~96.28% and relative standard deviations (RSD) of&#xa0;1.5%~2.18% for the samples. This study not only advances the current state of pesticide sensing technology but also establishes new pathways for the development of portable, low-power, and field-deployable monitoring systems. By integrating the advantages of semiconductor heterojunction engineering, photoelectrochemical modulation, and molecular recognition mechanisms, the OPECT platform provides a scalable and adaptable framework that can be readily extended to the detection of other toxic analytes.</p> Graphical Abstract <p></p>