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Obtaining single-phase tin monosulfide (SnS) films at low temperatures is challenging since cubic (π-SnS) and orthorhombic (α-SnS) polymorphs have similar energies of formation and grow under similar conditions. Here, we show that in atomic layer deposition (ALD) of polycrystalline SnS using tin­(II) acetylacetonate and H2S precursors, the substrate surface greatly influences the SnS phase evolution. For example, a silicon (100) substrate, with a highly hydroxylated surface, favors the growth of α-SnS. Meanwhile, ozone treatment or preannealing of the same substrate leads to mainly π-SnS. Just a few ALD cycles of another oxide or sulfide can even more substantially alter the outcome. Substrates that favor α-SnS growth typically produce initially enhanced growth rates, while those promoting π-SnS are partially surface-poisoned by the acetylacetonate precursor ligands. Growth of either polymorph is self-sustained after its initiation, and the sustaining factor appears to be the surface–ligand interaction; π-SnS preferentially evolves on substrates and π-SnS surfaces that are rich in highly reactive dangling bonds, while chemically inert substrates and α-SnS surfaces promote α-SnS. While lattice matching is less central, the role of ligand bonding in SnS ALD also helps explain the previously reported phase dependence on growth temperature and H2S precursor dose and shows promise for area-selective ALD of SnS.