We explore the phase behavior of room-temperature ionic liquids (RTILs) compressed under high pressure to determine whether they crystallize or hold a liquid state. RTILs have attractive supercooling properties compared with ordinary molecular liquids, which easily become a glassy state without crystallizing at ambient pressure. Thus, phase behavior under extreme stress, such as pressure, might yield interesting results. Here, we show that N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate ([DEME][BF4]) could be crystallized upon compression, but it usually formed a superpressed liquid. Alternatively, unusual crystallization could be induced by releasing the pressure on the superpressed liquid. Notably, crystal polymorphism was observed in the decompression process. These facts along with visual observations indicate the possibility of [DEME][BF4] serving as a superpressurized glass. Our findings may facilitate the development of a new range of applications for RTILs that have undergone high-pressure recrystallization.