Semiconducting two-dimensional crystals are currently receiving significant attention because of their great potential to be an ultrathin body for efficient electrostatic modulation, which enables to overcome the limitations of silicon technology. Here we report that, as a key building block for two-dimensional semiconductor devices, vertical p-n junctions are fabricated in ultrathin MoS₂ by introducing AuCl₃ and benzyl viologen dopants. Unlike usual unipolar MoS₂, the MoS₂ p-n junctions show ambipolar carrier transport, current rectification via modulation of potential barrier in films thicker than 8 nm and reversed current rectification via tunnelling in films thinner than 8 nm. The ultimate thinness of the vertical p-n homogeneous junctions in MoS₂ is experimentally found to be 3 nm, and the chemical doping depth is found to be 1.5 nm. The ultrathin MoS₂ p-n junctions present a significant potential of the two-dimensional crystals for flexible, transparent, high-efficiency electronic and optoelectronic applications.