Both pristine and Eu3+-doped scandium-potassium complex sulfate KSc(SO4)2 has been obtained with 99% yield via crystallization from solutions. The high-temperature XRPD and thermal studies revealed an enantiotropic first-order phase transition at 700–725 K and standard pressure. In the temperature region 724–754 K, the low-temperature stable monoclinic polymorph, β-KSc(SO4)2 (sp.gr. С2/m), coexists with the high-temperature trigonal modification, α-KSc(SO4)2 (sp.gr. P321), with decomposition at ∼1120 K. DFT calculations confirmed this rank of polymorphs and a low hospitality of their Sc-sublattices for Eu3+ cations. Under UV excitation, the KSc0.98Eu0.02(SO4)2 sample exhibits Eu3+ luminescence at 570−720 nm. Heating of the sample from 298 K to 423 K leads to a luminescence fading of 13% and to its a sharp decline at elevated temperatures. Kinetic studies have shown that the lifetime depends on the chosen excitation wavelength and temperature. An Х-band resonance in the sample of KSc(SO4)2:Eu3+ has been detected in the temperature range of 299–500 K. The EPR spectra are interpreted assuming the presence of sulfate radicals.