Changes in structure and phase transition behavior of (Na1/2Bi1/2)1−xSrxTiO3 were investigated using x-ray diffraction (XRD) analysis and dielectric measurements. A decrease in the degree of lattice distortion was observed when the structure was rhombohedral (x < 0.26). Further substitution of Sr2+ (x ≥ 0.26) caused (Na1/2Bi1/2)1−xSrxTiO3 to show no macroscopic lattice distortion. However, all investigated samples (0.1 ≤ x ≤ 0.5) were ferroelectric at room temperature regardless of the lack of lattice distortion found in XRD patterns. Further XRD investigations revealed that nonuniform strains caused by nonuniform polarized islands were responsible for this behavior. A-site cation substitution of Sr2+ also resulted in the decrease of Tmax (the temperature where dielectric constant is maximum). Typical relaxor ferroelectric phase transition behavior, associated with the shift of Tmax, with respect to the measuring frequencies, was found above x = 0.18. This could be also ascribed to the decrease of Tmax—this reveals the increase in the shift of Tmax with frequency, with associated decrease in ionic displacement and space charge contributions and associated increase in the relative contribution to the polarization by microdomains and domain walls at temperatures near Tmax.