Thickness-dependent structure–property relationships in strained SrRuO3 thin films on GdScO3 (GSO) substrates are reported. The film is found to have epitaxially stabilized crystal structures that vary with the film thickness. Below 16 nm, the √2apc × √2apc × 2apc monoclinic structure is stabilized while above 16 nm the film has the apc × 2apc × apc tetragonal structure. The thickness-dependent structural changes are ascribed to the substrate-induced modification in the RuO6 octahedral rotation pattern, which highlights the significance of the octahedral rotations for the epitaxial strain accommodation in the coherently-grown films. Close relationships between the structural and physical properties of the films are also found. The monoclinic film has the uniaxial magnetic easy axis 45° away from the GSO direction while the tetragonal film has the one that lies along the in-plane [1–10]GSO direction. The results demonstrate that the octahedral rotations in the strained perovskite oxide thin films are a key factor for determining their structure phases and physical properties.