In this Letter, we report on deep UV laser emitting at 249 nm based on thin GaN quantum wells (QWs) by optical pumping at room temperature. The laser threshold was 190 kW/cm2 that is comparable to state-of-the-art AlGaN QW lasers at similar wavelengths. The laser structure was pseudomorphically grown on a c-plane sapphire substrate by metalorganic chemical vapor deposition, comprising 40 pairs of 4 monolayer (ML) GaN QWs sandwiched by 6 ML AlN quantum barriers (QBs). The low threshold at the wavelength was attributed to large optical and quantum confinement and high quality of the material, interface, and Fabry-Pérot facet. The emissions below and above the threshold were both dominated by transverse electric polarizations thanks to the valence band characteristics of GaN. This work unambiguously demonstrates the potentials of the binary AlN/GaN heterojunctions for high-performance UV emitters.