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Abstract Control of BO₆octahedral rotations at the heterointerfaces of dissimilar ABO₃perovskites has emerged as a powerful route for engineering novel physical properties. However, its impact length scale is constrained at 2–6 unit cells close to the interface and the octahedral rotations relax quickly into bulk tilt angles away from interface. Here, a long‐range (up to 12 unit cells) suppression of MnO₆octahedral rotations in La_0.9Ba_0.1MnO₃through the formation of superlattices with SrTiO₃can be achieved. The suppressed MnO₆octahedral rotations strongly modify the magnetic and electronic properties of La_0.9Ba_0.1MnO₃and hence create a new ferromagnetic insulating state with enhanced Curie temperature of 235 K. The emergent properties in La_0.9Ba_0.1MnO₃arise from a preferential occupation of the out‐of‐plane Mnd_3z²_−r²orbital and a reduced Mn e_gbandwidth, induced by the suppressed octahedral rotations. The realization of long‐range tuning of BO₆octahedra via superlattices can be applicable to other strongly correlated perovskites for exploring new emergent quantum phenomena. 

Abstract Ultrathin epitaxial films of ferromagnetic insulators (FMIs) with Curie temperatures near room temperature are critically needed for use in dissipationless quantum computation and spintronic devices. However, such materials are extremely rare. Here, a room‐temperature FMI is achieved in ultrathin La_0.9Ba_0.1MnO₃films grown on SrTiO₃substrates via an interface proximity effect. Detailed scanning transmission electron microscopy images clearly demonstrate that MnO₆octahedral rotations in La_0.9Ba_0.1MnO₃close to the interface are strongly suppressed. As determined from in situ X‐ray photoemission spectroscopy, OK‐edge X‐ray absorption spectroscopy, and density functional theory, the realization of the FMI state arises from a reduction of Mn e_gbandwidth caused by the quenched MnO₆octahedral rotations. The emerging FMI state in La_0.9Ba_0.1MnO₃together with necessary coherent interface achieved with the perovskite substrate gives very high potential for future high performance electronic devices.