The lack of scalable-methods for the growth of 2D MoS2 crystals, an identified emerging material with applications ranging from electronics to energy storage, is a current bottleneck against its large-scale deployment. We report here a two-step ALD route with new organometallic precursors, Mo(NMe2)4 and 1,2-ethanedithiol (HS(CH2)2SH) which consists in the layer-by-layer deposition of an amorphous surface Mo(iv) thiolate at 50 °C, followed by a subsequent annealing at higher temperature leading to ultra-thin MoS2 nanocrystals (?20 nm-large) in the 1-2 monolayer range. In contrast to the usual high-temperature growth of 2D dichalcogenides, where nucleation is the key parameter to control both thickness and uniformity, our novel two-step ALD approach enables chemical control over these two parameters, the growth of 2D MoS2 crystals upon annealing being ensured by spatial confinement and facilitated by the formation of a buffer oxysulfide interlayer. © The Royal Society of Chemistry 2017.