In this paper, we describe a novel thermosensitive triblock copolymer in an ionic liquid (IL) that shows a low-temperature-sol–high-temperature-gel transition. A well-defined ABA triblock copolymer consisting of poly(benzyl methacrylate) as the terminal A blocks and poly(methyl methacrylate) as the middle B block (P(BnMA-b-MMA-b-BnMA), BMB) was successfully synthesized using atom transfer radical polymerization (ATRP) from a bifunctional initiator. The number-average molecular weights of the PBnMA blocks and the PMMA block were estimated to be 25 kDa and 33 kDa, respectively. The temperature dependence of the hydrodynamic radius obtained from dynamic light scattering showed that in dilute solutions (0.1 wt%) the triblock copolymer exhibited lower critical micellization temperature (LCMT)-type aggregation behaviour around 135 °C in a common hydrophobic IL, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][NTf2]). Dynamic viscoelastic measurements for a 20 wt% BMB solution in [C2mim][NTf2] confirmed that a viscous liquid at low temperature (G′ (storage elastic modulus) < G′′ (loss elastic modulus)) becomes a gel (ion gel) (G′ > G′′) upon heating above the aggregation temperature of the PBnMA terminal blocks. No gelation was observed when the triblock copolymer concentration was below 10 wt%. Furthermore, the thermoreversible ion gel exhibited excellent sol–gel transition reversibility for multiple heating/cooling cycles performed over several days.