<p>Various concentrations (0.02, 0.04, 0.06, and 0.08&#xa0;mol) of Sm³⁺-doped CaCO₃/PEG nanoparticles were first synthesized from a well-known dolomite source using a biomimetic method. The presence of all the XRD planes agreed with the standard value of the rhombohedral structure of calcite. The average dislocation density, microstrain, and crystallite sizes of the prepared products were calculated, and the products are in the nanometer range (29–35&#xa0;nm). The characteristic FTIR peaks also confirmed the presence of the calcite phase of the D_3h polymorph and metal oxide (Sm–O) by the appearance of a peak around 529&#xa0;cm⁻¹. The doping of Sm³⁺ in CaCO₃/PEG nanocomposites was also assessed by XPS, FE-SEM with EDX mapping, and UV-visible DRS spectra. 0.02&#xa0;mol Sm³⁺-doped CaCO₃/PEG nanoparticles showed a red shift compared with the other three concentrations. The observed emission values are in the blue band region. 0.02&#xa0;mol Sm³⁺-doped CaCO₃/PEG exhibited an aggregated spherical morphology. It was subjected to photocatalytic analysis under UV light illumination against three major dyes: methylene blue (MB), crystal violet (CV), and Congo red (CR). The calculated degradation efficiencies of MB, CV, and CR were 95, 92, and 89%, respectively. The chemical interactions of MB, CV, and CR with nCS₁ are illustrated. A possible mechanism for the photoexcitation of 0.02&#xa0;mol Sm³⁺-doped CaCO₃/PEG nanoparticles is proposed. The factors affecting the degradation efficiency, the role of scavengers, recyclability, and stability of the photocatalyst were effectively analyzed. After two cycles of usage, the degradation efficiency of the MB by 0.02&#xa0;mol Sm³⁺-doped CaCO₃/PEG nanoparticles was maintained at 92%. Thus, the present 0.02&#xa0;mol Sm³⁺-doped CaCO₃/PEG nanoparticles photocatalyst could be effective in degrading major dyes in wastewater.</p>