Thermoresponsive polymers that possess both UCST- and LCST-like behaviors have generally been designed using diblock copolymers that are mostly composed of an LCST-like polymer and a UCST polymer. Herein, we prepared an LCST-UCST-type polymer composed of UCST-like thermoresponsive zwitterionic sulfabetaine methacrylate and nonthermoresponsive PEG methacrylate, ZB-PEG, by reversible addition-fragmentation chain transfer (RAFT) copolymerization. By adjusting the PEG composition, ZB-PEG formed a mesoglobule, a microglobule, and the dissociated states in phosphate-buffered saline (PBS). These states were found to be reversible via temperature control. Moreover, this behavior showed high reversibility and succeeded in stabilizing horseradish peroxidase (HRP) in the dilute condition. Such thermoresponsive ZB-PEG can be applied over a wide range of applications in biotechnology and other fields.