Two-dimensional (2D) tungsten disulfide (WS2) has emerged as a promising ultrathin semiconductor for building high-performance nanoelectronic devices. The controllable synthesis of WS2 monolayers (1L) with both large size and high quality remains as a challenge. Here, we developed a new approach for the chemical vapor deposition (CVD) growth of WS2 monolayers by using K2WS4 as the growth precursor. The simple chemistry involved in our approach allowed for improved controllability and a fast growth rate of ~ 30 μm·min−1. We achieved the reliable growth of 1L WS2 flakes with side lengths of up to ~ 500 μm and the obtained WS2 flakes were 2D single crystals with low density of defects over a large area as evidenced by various spectroscopic and microscopic characterizations. In addition, the large 1L WS2 single crystals we obtained showed higher electrical performance than their counterparts grown with previous approaches, demonstrating the potential of our approach in producing high quality and large 2D semiconductors for future nanoelectronics.