An important concern in the deposition of thin hydrogenated amorphous silicon (a-Si∶H) films is to obtain smooth surfaces. Herein, we combine molecular-dynamics simulations with first-principles density functional theory calculations to elucidate the smoothening mechanism of plasma deposited a-Si∶H thin films. We show that the deposition precursor may diffuse rapidly on the a-Si∶H film surface via overcoordinated surface Si atoms and incorporate into the film preferentially in surface valleys, with activation barriers for incorporation dependent on the local surface morphology. Experimental data on smoothening and precursor diffusion are accounted for.