A novel monoclonal antibody (MAb) specific to the seedborne bacterium Acidovorax citrulli was produced. MAb 11E5 reacted specifically with 19 strains of A. citrulli but not with three closely related bacteria in the family Comamonadaceae (i.e., A. facilis, Comamonas acidovorans, and C. testosteroni) and another seven phytopathogenic bacteria. Moreover, this MAb detected a strain of A. citrulli that was not detected by a commercial enzyme-linked immunosorbent assay (ELISA)-based kit and a commercial immunochromatographic strip test. In Western blot analysis, MAb 11E5 reacted with an A. citrulli protein of a molecular mass >170 kDa. MAb 11E5 was employed to develop two sandwich ELISA systems: MAb captured-sandwich ELISA (MC-sELISA) and polyclonal antibody captured-sandwich ELISA (PC-sELISA). MC-sELISA was 10 times more sensitive than PC-sELISA for detection of A. citrulli in cucurbit leaf and seed extracts. The detection limit of the MC-sELISA was 5 × 104 CFU/ml. Detection of A. citrulli in naturally infected cucurbit leaves, fruit, and seed was also feasible using MC-sELISA. The newly established MC-sELISA provides another alternative for specific detection of A. citrulli in cucurbits and can be applied for routine field inspection.