A sensitive and stable surface plasmon resonance (SPR) immunosensor based on the inhibition format was developed and validated for detecting melamine (MEL) in milk products and pet foods. The sensitivity and the limit of detection (LOD) of the proposed method for MEL were 2.32 × 10(-2) and 1.4 × 10(-3) μg/mL, respectively. The immunosensor was highly specific to MEL, which displayed only low cross-reactivity (CR) (<0.01%) for cyanuric acid, cyanuric chloride, and atrazine. The assay was validated for the detection of MEL in full-cream milk, skim milk powder, infant formula, dog food, and cat food. Most of the recovery results ranged between 76 and 115%. The sensitivities of the assay in each type of sample were 2.57 × 10(-2) μg/mL, 2.32 × 10(-2) μg/kg, 2.51 × 10(-2) μg/kg, 2.66 × 10(-2) μg/kg, and 2.68 × 10(-2) μg/kg, respectively, which were much lower than the maximum residue levels (MRLs) of MEL.