Unmodified screen-printed boron-doped diamond electrodes (SPE-BDD) were systematically evaluated for their suitability in the electrogenerated chemiluminescence (ECL) detection of As(III) using luminol as the luminophore. The sensor was designed based on a co-reactant ECL pathway, wherein hydrogen peroxide (H₂O₂) functioned as a co-reactant to amplify the luminol ECL signal. Cyclic voltammetry (CV) was utilized to elucidate the electrochemical properties of luminol and to investigate the impact of As(III) on the ECL response. Meanwhile, emission signals were recorded via a photosensor module simultaneously. Experimental parameters, including solution pH, H2O2and luminol concentrations, and scan rate, were thoroughly optimized to enhance sensor performance. Although SPE-BDD is unreactive to As(III), analytical detection was successfully achieved through luminol emission change. The presence of As(III) produced a concentration-dependent quenching of the ECL signal under alkaline conditions, enabling sensitive detection across a wide dynamic range. The sensor achieved a detection limit of 41 nM and demonstrated excellent repeatability (RSD = 0.67 %), highlighting its sensitivity and reliability for As(III) detection. Notably, the use of unmodified SPE-BDD electrodes eliminates the need for surface modification, offering a simplified yet effective sensing platform. The optimum buffer solution pH of 10 contributes significantly to its excellent selectivity by promoting optimal deprotonation of luminol and maintaining arsenic in its dissolved state, while other potential metal ion interferences tend to form solid metal (hydro)oxide. The proposed sensor was successfully applied to the detection of As(III) in a seawater matrix, confirming its feasibility and robustness for environmental monitoring applications. © 2025 Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.