Aptamers that recognize specific cells in a complex environment have emerged as invaluable molecular tools in bioanalysis and in the development of targeted therapeutics. The selective recognition of aptamers, however, can be compromised by the coexistence of target receptors on both target cells and other cells. To address this problem, we constructed a structure-switchable aptamer (SW-Apt) with reconfigurable binding affinity in accordance with the microenvironment of target cells. The SW-Apt makes use of i-motifs, which are quadruplex structures that form in sequences rich in cytosine. More specifically, we report the design of single-stranded, pH-responsive i-motif-modified aptamers able to bind specifically with target cells by exploiting their pH. Here, the i-motif serves as a structural domain to either facilitate the binding ability of aptamers to target cells or suppress the binding ability of aptamers to nontarget cell based on the pH of the cellular microenvironment. SW-Apt exhibited high binding ability with target cells at acidic pH, while no obvious binding was observed at physiological pH. The i-motif-induced structure-switching was verified with Förster resonance energy transfer and circular dichroism spectroscopy. Notably, SW-Apt exhibits high specificity in serum and excellent stability, likely attributed to the folded quadruplex i-motif structure. This study provides a simple and efficient strategy to chemically modulate aptamer binding ability and thus improve aptamer binding specificity to target cells, irrespective of the coexistence of identical receptors on target and nontarget cells.