Ultrathin two-dimensional (2D) nanomaterials have attracted intense research efforts due to their extraordinary optoelectronic properties. However, the nucleation and growth mechanisms of 2D colloidal nanosheets are still poorly understood. Here, we follow the formation of ultrathin colloidal Cu2-xS nanosheets by in situ small-angle X-ray scattering. While thermal decomposition of copper-dodecanethiolates produces spheroidal Cu2-xS nanocrystals, the addition of chloride to the reaction mixture results in 2 nm thick Cu2-xS nanosheets with well-defined shape and size. Our results show that chloride stabilizes stacks of lamellar copper-thiolate supramolecular complexes, so that they remain intact beyond the onset of Cu2-xS nucleation at 230 °C, leading to 2D-constrained stack-templated nucleation and growth. The face-to-face stacking of the nanosheets reinforces the 2D constraints imposed by the lamellar soft template, since it prevents internanosheet mass transport and nanosheet coalescence, thereby inhibiting growth in the thickness direction and allowing only for lateral growth. Our work thus provides novel insights into soft-templating formation mechanisms of ultrathin colloidal nanosheets, which may be exploited for other metal sulfide compositions.