Protonolysis reactions of [(Me₃Si)₂N]₃Ln(μ-Cl)Li(THF)₃ (Ln = Pr, Nd, Sm, Eu) with 3 equiv of 4,6-dimethylpyrimidine-2-thiol (dmpymtH) gave rise to the four Ln(III) pyrimidine-2-thionate complexes Li[Ln(dmpymt)₄] (Ln = Pr (1), Nd (2), Sm (3), Eu (4)). Compounds 1−4 were characterized by elemental analysis, IR and ¹H NMR spectroscopy, and single-crystal X-ray diffraction. X-ray diffraction analysis shows that the structures of 1−4 are similar and each eight-coordinate Ln(III) ion is chelated by four dmpymt ligands. Complexes 1−4 display excellent catalytic performance in the cyclodimerization of isocyanates to produce substituted ureas via elimination of CO, which represents the first example of lanthanide thiolates exhibiting a high catalytic activity and a high selectivity in the cyclodimerization of isocyanates. The effects of the solvents, temperatures, catalyst loadings, and rare-earth metals on the catalytic activities of the complexes were examined.