We present a study of KTaO₃:Er (≈0.05%) single crystals based on a multifaceted approach including the use of optical absorption, far-infrared reflectivity, electron paramagnetic resonance, photoluminescence spectra, and ab initio simulations. We describe briefly the fundamental consequences of Er doping, in particular, stiffening of TO1 soft mode of KTaO₃. We provide information about energy level structures controlling f–f optical transitions in Er³⁺, on formation of minor cubic octahedral and major orthorhombic Er³⁺ centers. It is revealed that the temperature shift of narrow zero-phonon emission lines is strikingly unusual being much larger than that typical for trivalent rare-earth impurities.