The development of convenient and highly sensitive methodologies for evaluating drug levels in serum is crucial for clinical diagnosis. As the most prescribed drug against hyperthyroidism, methimazole (MMZ) lacks the simple and rapid quantitative method. Herein, an electrochemiluminescent (ECL) sensor is proposed for MMZ detection using an aggregation-induced delayed ECL (AIDECL) active organic nanodots (ODs) as a probe. Due to the full utilization of excited species, these ODs exhibit high ECL efficiency. In the presence of MMZ, such sensor exhibits decreased ECL signal of ODs since MMZ can consume co-reactants' radical cations that are vital mediates for ECL emission of ODs. Consequently, the developed sensor demonstrates excellent performance for the analysis of MMZ with a low detection limit of 7 × 10