We demonstrate the tunability of the photoluminescence (PL) properties of monolayer (1L)-MoS2 via chemical doping. The PL intensity of 1L-MoS2 was drastically enhanced by the adsorption of p-type dopants with high electron affinity but reduced by the adsorption of n-type dopants. This PL modulation results from switching between exciton PL and trion PL depending on carrier density in 1L-MoS2. Achievement of the extraction and injection of carriers in 1L-MoS2 by this solution-based chemical doping method enables convenient control of optical and electrical properties of atomically thin MoS2.