Controlled substitutional doping of two-dimensional transition-metal dichalcogenides (TMDs) is of fundamental importance for their applications in electronics and optoelectronics. However, achieving -type conductivity in MoS and WS is challenging because of their natural tendency to form -type vacancy defects. Here, we report versatile growth of -type monolayer WS by liquid-phase mixing of a host tungsten source and niobium dopant. We show that crystallites of WS with different concentrations of substitutionally doped Nb up to 10 cm can be grown by reacting solution-deposited precursor film with sulfur vapor at 850 °C, reflecting the good miscibility of the precursors in the liquid phase. Atomic-resolution characterization with aberration-corrected scanning transmission electron microscopy reveals that the Nb concentration along the outer edge region of the flakes increases consistently with the molar concentration of Nb in the precursor solution. We further demonstrate that ambipolar field-effect transistors can be fabricated based on Nb-doped monolayer WS.