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The origin of insulating ferromagnetism in epitaxial LaCoO 3 films under tensile strain remains elusive despite extensive research efforts are devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, the spin state in epitaxial LaCoO 3 thin films is systematically investigated to clarify the mechanism of strain-induced ferromagnetism using element-specific X-ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, it is unambiguously demonstrated that Co 3+ in LaCoO 3 films under compressive strain (on LaAlO 3 substrate) is practically a low-spin state, whereas Co 3+ in LaCoO 3 films under tensile strain (on SrTiO 3 substrate) have mixed high-spin and low-spin states with a ratio close to 1:3. From the identification of this spin state ratio, it is inferred that the dark strips observed by high-resolution scanning transmission electron microscopy indicate the position of Co 3+ high-spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO 3 films. This consequently explains the nature of ferromagnetism in LaCoO 3 films. The study highlights the importance of spin state degrees of freedom, along with thin-film strain engineering, in creating new physical properties that do not exist in bulk materials., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)