Associative phase separation (complex coacervation) in liquid-liquid phase separation (LLPS) involves the separation of multiple substances into concentrated and dilute phases by electrostatic interactions. Simple phase separation (simple coacervation) occurs when the hydrophilicity and hydrophobicity of a single molecule change dramatically in response to a specific stimulus. Simple coacervation arises from the lower critical solution temperature (LCST)- and upper critical solution temperature (UCST)-type phase separations in aqueous media containing temperature-responsive polymers. LCST- and UCST-type LLPSs induce droplet formation at temperatures above the LCST and below the UCST, respectively. Although there have been several studies on the UCST-type LLPS of temperature-responsive polymers in water, only a few temperature-responsive polymers that exhibit UCST-type LLPS in aqueous media with physiological ionic strength have been reported. In this study, we synthesized temperature-responsive zwitterionic polymers, exhibiting UCST-type LLPS in physiological ionic strength, by copolymerizing two types of zwitterionic monomers─sulfabetaine (SaB) having ammonium and sulfate groups and sulfobetaine (SB) having ammonium and sulfonate groups─in aqueous media with various ionic strengths. The resulting zwitterionic copolymers, P(SaB-