The first lanthanide complexes with a redox-active sulfur diimide ligand, [LnCp*2(Me3SiN=)2S] (Ln=Sm, Eu, Yb; Cp*=η5-C5Me5), are reported. The complexes were synthesized by using [LnCp*2(THF)2] and (Me3SiN=)2S and have been thoroughly characterized by single-crystal X-ray diffraction, EPR spectroscopy, UV/Vis/NIR electronic absorption spectroscopy and SQUID magnetometry. The results, as interpreted by CASSCF/SOC-RASSI calculations providing a non-perturbative treatment of spin-orbit coupling, indicate that these paramagnetic complexes are best described as Ln3+ and [(Me3SiN=)2S]− . adducts. As such, these complexes contain the first isolated and structurally characterized acyclic [(RN=)2S]− . radical anions.