Plasmon lasers are a new class of coherent optical amplifiers that generate and sustain light well below its diffraction limit(1-4). Their intense, coherent and confined optical fields can enhance significantly light-matter interactions and bring fundamentally new capabilities to bio-sensing, data storage, photolithography and optical communications(5-11). However, metallic plasmon laser cavities generally exhibit both high metal and radiation losses, limiting the operation of plasmon lasers to cryogenic temperatures, where sufficient gain can be attained. Here, we present a room-temperature semiconductor sub-diffraction-limited laser by adopting total internal reflection of surface plasmons to mitigate the radiation loss, while using hybrid semiconductor-insulator-metal nanosquares for strong confinement with low metal loss. High cavity quality factors, approaching 100, along with strong lambda/20 mode confinement, lead to enhancements of spontaneous emission rate by up to 18-fold. By controlling the structural geometry we reduce the number of cavity modes to achieve single-mode lasing.