We investigated pressure effects on the lower critical solution temperature (LCST)-type phase behavior of a thermoresponsive polymer in an ionic liquid (IL) solution. The cloud point temperature of the IL solution increased monotonically with pressure, which was quite different from that of conventional aqueous polymer solutions reported in previous work, which exhibited a convex-upward-type pressure dependence. We compared the IL and aqueous systems and concluded that the difference results from their solvation mechanisms. Dynamic light scattering (DLS) measurements showed an appearance of a slow mode (corresponding to aggregation) in addition to the fast mode (corresponding to molecular dispersion) at the cloud point pressure, indicating an onset of pressure-induced phase separation. This work contributes to the fundamental understanding of the phase behavior of polymers in IL systems under high pressure.