Large area and uniform monolayer ${\mathrm{MoS}}_2$ is of great importance for optoelectronic devices but is commonly suffering from rather weak photoluminescence. Here, by engineering the concentration profiles of gaseous chemicals through extra trace amounts of water, we demonstrate the uniform dendrite-type growth of monolayer ${\mathrm{MoS}}_2$ unraveled by spatially resolved fluorescence spectroscopy, which exhibits macroscopic monolayer flakes (up to centimeter scale) with photoluminescence intensity of orders of magnitude higher than conventional chemical vapor deposition monolayer ${\mathrm{MoS}}_2$. Both spectroscopic evidence and theoretical models reveal that the fast-fractal dendrite growth can be ascribed to the extra introduced water sources that generate sufficient aqueous gas around the S-poor regions nearby the central-axis zone, leading to highly efficient Mo sources transport, accelerated S atom corrosion nearby grain edges, and/or defect sites, as well as enhanced photoemission intensity. Our results may provide new insight for high throughput fabrication of ${\mathrm{MoS}}_2$ monolayers with high yield photoluminescence efficiency.