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Several applications of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) in nanoelectronic devices require the deposition of ultrathin pinhole free high-κ dielectric films on 2D TMDs. However, deposition of nm-thin high-κ dielectric films on 2D TMDs remains challenging due to the inert TMD surface. Here, we demonstrate that the surface of a synthetic polycrystalline 2D MoS2film is functionalized with SiO2to enable the atomic layer deposition (ALD) of thin and continuous Al2O3and HfO2layers. The origins of nucleation, the growth mode, and layer coalescence process have been investigated by complementary physical characterization techniques, which can determine the chemical bonds, absolute amount, and surface coverage of the deposited material. SiO2is prepared by oxidizing physical vapor deposited Si in air. The surface hydrophilicity of MoS2significantly increases after SiO2functionalization owing to the presence of surface hydroxyl groups. SiO2layers with a Si content of only 1.5 × 1015atoms/cm2enable the deposition of continuous 2 nm thin Al2O3and HfO2layers on MoS2at 300 °C. This fast layer closure can be achieved despite the sub-nm thickness and discontinuity of the SiO2nucleation layer. On the basis of the experimental results, we propose a nucleation mechanism that explains this fast layer closure. Nucleation of Al2O3and HfO2occurs on the SiO2islands, and fast layer closure is achieved by the lateral growth starting from the many nm-spaced SiO2islands. Finally, the dielectric properties of Al2O3on the functionalized MoS2are confirmed in a top-gated capacitor that shows a leakage current of 3.8 × 10–6A/cm2at a 3.4 nm equivalent oxide thickness. To conclude, fast nucleation and layer closure in ALD can be achieved even for a sub-nm thin, discontinuous nucleation layer. We propose that this insight can also be applied to other ALD processes, materials, or applications where thin and fully continuous layers are required.