Continuous-wave (CW) lasing at room temperature is critical for integration with opto-electronic devices and optimal modulation of optical interactions. Plasmonic nanocavities integrated with gain can generate coherent light at sub-wavelength scales, while insufficient gain with respect to losses and thermal instabilities in nanocavities has limited all nanoscale lasers to pulsed pump sources and/or low-temperature operation. We developed a materials platform to realize CW upconverting lasing at room temperature with record-low thresholds (70 W/cm2) and high photostability from sub-wavelength plasmons. Single-mode lasing was achieved from upconverting nanoparticles conformally coated on Ag nanopillar arrays that support a single, sharp lattice plasmon cavity mode. Our plasmon-nanoarray upconverting lasers provide directional, ultra-stable output at visible frequencies under near-infrared pumping, even after six hours of constant operation, which offers prospects in previously unrealizable applications of coherent nanoscale light.