The authors report the device characteristics of GaInSb/AlGaSb quantum well (QW) lasers monolithically grown on GaAs substrates. The 7.8% lattice mismatch between GaAs substrates and GaSb buffer layers can be completely accommodated by using an interfacial misfit (IMF) array. Room-temperature lasing operation is obtained from a 1.25-mm-long device containing six-layer Ga0.9In0.1Sb/Al0.35Ga0.65Sb QWs at 1.816 μm with a threshold current density of 1.265 kA/cm2. The observed characteristic temperature and temperature coefficient are 110 K and 9.7 Å/K, respectively. This IMF technique will enable a wide range of lasing wavelengths from near-infrared to midwavelength-infrared regimes on a GaAs platform.