Potentiometric immunoassays have been utilized for the quantitative detection of alpha-fetoprotein (AFP) in hepatocellular carcinoma, but most of them involve low sensitivity and enzyme labels, and thus are unfavorable for routine use. In this work, we report the proof-of-concept of a sensitive and powerful ion-selective potentiometric sensing method for AFP detection with an in situ amplified signal readout. This potentiometric immunoassay mainly contains a silver nanocluster-functionalized single-stranded DNA (AgNC-DNA), a short DNA primer and two antibodies. A sandwich-type immunoreaction is employed for AFP determination on an anti-AFP capture antibody-coated microplate using a biotinylated human AFP secondary antibody. Coupling with a typical biotin-avidin system, the biotinylated DNA initiator strands are conjugated on the microplate in the presence of AFP to induce the rolling cycle amplification (RCA) reaction, followed by AgNC-DNA hybridization. Upon addition of HNO3, the hybridized AgNCs are dissolved into numerous Ag(I) ions, which can be readily determined on a portable handheld silver-ion selective electrode (Ag-ISE). Under optimal conditions, the electrode potential increases with an increase in AFP concentration and exhibits a good linear range of 0.01-100 ng mL-1 at a detection limit of 7.9 pg mL-1. Moreover, the Ag-ISE-based potentiometric immune assay also shows good reproducibility, high specificity and long-term storage stability. Importantly, 18 human serum specimens containing the AFP analyte are screened using the potentiometric immunoassay, giving well-matched experimental results relative to the referenced enzyme-linked immunosorbent assay (ELISA) method.