2015 SPIE. The concept of parity-time (PT) symmetry exploits the interplay between the material loss and gain to attain novel optical phenomena such as exceptional point and unidirectional light propagation. Here we experimentally demonstrate a PT symmetry breaking laser that allows unique control of the resonant modes. In contrast to conventional ring cavity lasers with multiple competing modes, our on-chip InGaAsP/InP based PT microring laser exhibits intrinsic single-mode lasing regardless of the gain spectral bandwidth. Thresholdless parity-time symmetry breaking due to the rotationally symmetric structure leads to stable single-mode operation at the specific whispering gallery mode order.