The screening of glycoprotein markers has become an integral part of the in vitro diagnosis of malignant tumors. Herein, an electrochemical method based on alkaline phosphatase (ALP)-mediated enzymatic silver deposition is reported for the highly sensitive detection of glycoprotein tumor markers, in which ALP enzymes are decorated to the glycan moieties of targets via the lectin-carbohydrate interactions. As glycoproteins are conjugated with multiple glycan chains, lectin-mediated labeling can result in the decoration of each target with multiple ALP enzymes. Moreover, the enzymatic hydrolysis of ascorbic acid 2-phosphate into ascorbic acid can result in the deposition of a high density of silver particles, which can then be sensitively assayed via the robust Ag/AgCl solid-state voltammetric process. As a result, the enzymatic silver deposition-based electrochemical method exhibits high sensitivity. Using the aptamer-based electrochemical detection of the breast cancer-associated glycoprotein CA15-3 as a proof of concept, a detection limit of 0.32 mU/mL has been demonstrated. Results show that the synergism of the aptamer-based capture and the glycoform-specific discrimination capability of the lectin-carbohydrate interactions can endow this method with high selectivity, and its practical use in the assay of CA15-3 levels in serum samples has been illustrated. With benefits from the high efficiency, mild reaction conditions, and user-friendly operation, the lectin-mediated labeling of ALP enzymes for enzymatic silver deposition is highly applicable to the electrochemical detection of low-abundance glycoprotein tumor markers.