The controllable synthesis of MoS₂ monolayers is the key challenge for their practical applications. Here we report the chemical vapor deposition (CVD) growth of single layered MoS₂ by utilizing a bifunctional precursor. This precursor is a metal–organic complex which supplies both Mo sources and organic seeding promoters for the efficient CVD growth of MoS₂ monolayers. The successful growth of high quality MoS₂ flakes indicates that the rational design of bifunctional precursors will open up a new way for the controllable CVD growth of two-dimensional (2D) transition metal dichalcogenides (TMDCs).