A streptavidin-functionalized silver-nanoparticle-enriched carbon nanotube (CNT/Ag NP) is designed as trace tag for ultrasensitive multiplexed measurements of tumor markers using a disposable immunosensor array. The CNT/Ag NP nanohybrid is prepared by one-pot in situ deposition of Ag NPs on carboxylated CNTs. The nanohybrid is functionalized with streptavidin via the inherent interaction between the protein and Ag NPs for further linkage of biotinylated signal antibodies to obtain tagged antibodies. The functionalization process greatly improves the dispersibility of the nanohybrid in water. The immunosensor array is prepared by covalently immobilizing capture antibodies on chitosan-modified screen-printed carbon electrodes. Through a sandwich-type immunoreaction on the immunosensor array, numerous Ag NPs are captured onto every single immunocomplex and are further amplified by a subsequent Ag NP-promoted deposition of silver from a silver enhancer solution to obtain the sensitive electrochemical-stripping signal of the Ag NPs. Using carcinoembryonic antigen and α-fetoprotein as model analytes, this proposed multiplexed immunoassay method shows acceptable precision and wide linear ranges over four orders of magnitude with detection limits down to 0.093 and 0.061 pg mL−1, respectively. The assay results of serum samples with the proposed method are in acceptable agreement with the reference values. The newly designed strategy and the functionalized tag avoid cross-talk and the requirement of deoxygenation for electrochemical immunoassay, and thus provide a promising potential in clinical application.