The Brønsted acid, trifluoromethanesulfonimide (HNTf₂), was revealed to act as a highly efficient activator for group transfer polymerization (GTP) of methyl methacrylate (MMA) at ambient temperature. The HNTf₂-catalyzed GTP of MMA initiated by 1-methoxy-1-trimethylsilyloxy-2-methyl-propene (MTS) proceeded in a living manner to produce poly(methyl methacrylate)s (PMMA) with controlled molecular weights and narrow molecular weight distributions. Kinetic measurement and postpolymerization experiment further supported that the polymerization proceeded in a living fashion. Furthermore, from the MALDI-TOF MS analysis, only one series of peaks was observed, which was in perfect agreement with the molecular weight of PMMA possessing the MTS initiator residue and the desilylated chain end, showing that the HNTf₂-catalyzed GTP of MMA proceeded without any side reactions. The syndiotacticity of the PMMA obtained by the HNTf₂-catalyzed GTP increased with the decreasing polymerization temperature; the syndiotactic (rr) triad content was 72% at 27 °C and 90% at −55 °C.