Grain boundaries have a major effect on the physical properties of two-dimensional layered materials. Therefore, it is important to develop simple, fast and sensitive characterization methods to visualize grain boundaries. Conventional Raman and photoluminescence methods have been used for detecting grain boundaries; however, these techniques are better suited for detection of grain boundaries with a large crystal axis rotation between neighbouring grains. Here we show rapid visualization of grain boundaries in chemical vapour deposited monolayer MoS2 samples with multiphoton microscopy. In contrast to Raman and photoluminescence imaging, third-harmonic generation microscopy provides excellent sensitivity and high speed for grain boundary visualization regardless of the degree of crystal axis rotation. We find that the contrast associated with grain boundaries in the third-harmonic imaging is considerably enhanced by the solvents commonly used in the transfer process of two-dimensional materials. Our results demonstrate that multiphoton imaging can be used for fast and sensitive characterization of two-dimensional materials.