We synthesized and characterized for the first time an ampholytic ion-recognition linear copolymer of [3-(methacryloylamino)propyl]trimethylammonium chloride (MAPTAC), acrylic acid (AA), and benzo[18]crown-6-acrylamide (BCAm). In this copolymer, the MAPTAC unit has a positive charge. The AA unit has a negative charge that depends on the pH. The crown receptor of the BCAm unit forms a complex with specific ions such as Ba2+ because of the high complex formation constant which behaved like a fixed positive charge. Thus, the copolymers behaved as an ion-recognition polyampholytes and shrank at a pH equal to the isoelectric point (IEP), which shifted to a higher pH when the BCAm complexed with a cation. At that time, the BCAm also became hydrophilic with water of hydration accompanied by the cation. As a result of the combination of these two effects, we found that the reverse behaviors of swelling and shrinking occurred at different pHs in response to the same ion signal.