Metastable inorganic materials with unique properties are important in many practical applications, but their synthesis is often challenging. In physics, epitaxial stabilization, also known as pseudomorphic growth, is used to synthesize metastable polymorphs, but usually only as very thin films and on expensive single-crystal substrates. In chemistry, the templated growth of inorganic solid-state materials on self-assembled monolayers of organic molecules is reported. Bridging these two fields, here, we show that the synthesis of metastable polymorphs is possible up to large film thickness on amorphous substrates covered with thin inorganic seed layers that serve as templates. The stabilization of a 500-nm-thick metastable wurtzite (WZ) MnTe film by a 5-nm-thick ZnTe seed layer sputtered on amorphous glass substrates is experimentally demonstrated. Theoretical calculations explain this experimental observation by the small WZ polymorph energy relative to that of the ground-state nickeline (NC) structure of MnTe and a large lattice constant difference of the two. The resulting metastable WZ-MnTe polymorph exhibits a wide band gap of 2.7 eV and a low hole density of 10¹²cm⁻³, which is relevant to optoelectronic applications. These properties are in sharp contrast to those of the narrow-band-gap highly doped NC-MnTe with 1.3 eV band gap and 10¹⁹cm⁻³ hole density. The difference in hole density is due to the calculated difference in the formation energy of manganese vacancy acceptor defects. Overall, these results suggest that templated growth on amorphous substrates with seed layers can be used to synthesize metastable polymorphs of other materials, without the need for expensive single-crystal substrates.