Abstract: Abscisic acid (ABA) is one of the most important endogenous hormones to regulate seed dormancy, stomatal movement, root architecture, and plant responses to abiotic stresses. Therefore, it is often required to rapidly and accurately monitor ABA in living leaves for crop stress status in precision agriculture. However, conventional high-performance liquid chromatography, liquid chromatography–mass spectrometry, and enzyme-linked immunosorbent assay generally require the type-II band alignment, resulting in suppression of electron-hole recombination. A signal-enhanced PEC sensing interface has been constructed to immobilize an ABA-specific aptamer on the BiOI/g-C₃N₄/ITO electrode. The binding between ABA and aptamer has induced conformational rearrangement of the recognition layer, reduced the interfacial charge-transfer resistance, and further promoted electron transport, leading to an increased cathodic photocurrent. However, the aptasensor shared a better linear relationship between photocurrent intensity and the logarithm of sample pretreatment, expensive instruments, and laboratory operation, thereby limiting their use for on-site and dynamic analysis. In this study, a photoelectrochemical (PEC) aptasensor was developed for in-situ detection of ABA in tomato leaves under salt stress, according to a bismuth oxyiodide/graphitic carbon nitride (BiOI/g-C₃N₄) type-II heterojunction. BiOI was selected as the visible-light-responsive semiconductor, while g-C₃N₄ was introduced as the carrier and photosensitizer to improve the separation and transfer of photogenerated charge carriers. The BiOI/g-C₃N₄ composite was prepared by simple physical mixing and stirring. The sample was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and ultraviolet-visible diffuse reflectance spectroscopy. The results showed that g-C₃N₄ nanosheets were tightly coupled with flower-like BiOI microspheres, indicating an effective heterojunction interface with enhanced visible-light absorption and interfacial charge transport. Compared with pure BiOI, the BiOI/g-C₃N₄-modified electrode exhibited a 13-fold higher photocurrent response, which was attributed to ABA concentration in the range of 1 pmol/L to 300 nmol/L, with a correlation coefficient of 0.992 and a detection limit of 0.33 pmol/L. The sensor also exhibited high selectivity against common coexisting substances, including inorganic ions, amino acids, sugars, and other plant hormones, even when the interferents were present at 100-fold higher concentrations. In addition, stable responses were maintained in the temperature range of 15-35 °C, and 91.8% of the initial signal was retained after 14 days of storage at 4 °C. Recovery tests showed that tomato leaf extracts shared recoveries of 93%-106% with relative standard deviations of 1.1%-2.1%, indicating acceptable accuracy in complex plant matrices. Finally, the PEC sensing system was applied to the in-situ monitoring of ABA in tomato leaves during salt stress. The ABA level increased rapidly at the early stage of stress, reached 0.896 nmol/L at 24 h, and then decreased to 0.417 nmol/L at 72 h. This trend was consistent with the physiological role of ABA in early osmotic stress adaptation and subsequent metabolic adjustment. The BiOI/g-C₃N₄ PEC aptasensor can provide a sensitive, selective, and practical approach for dynamic ABA monitoring in plant leaves. The findings can offer a useful sensing strategy for crop stress diagnosis in precision agriculture.