The reactions of the tetrasiloxide U(III) complexes [U(OSi(O^tBu)₃)₄K] and [U(OSi(O^tBu)₃)₄][K18c6] with 0.5 equiv. of triphenylphosphine sulfide led to reductive S-transfer reactions, affording the U(IV) sulfide complexes [SU(OSi(O^tBu)₃)₄K₂]₂, 1, and [{SU(OSi(O^tBu)₃)₄K₂}₂(μ-18c6)], 2, with concomitant formation of the U(IV) complex [U(OSi(O^tBu)₃)₄]. Addition of 1 equiv. of 2.2.2-cryptand to complex 1 resulted in the isolation of a terminal sulfide complex, [SU(OSi(O^tBu)₃)₄K][Kcryptand], 3. The crucial role of the K⁺ Lewis acid in these reductive sulfur transfer reactions was confirmed, since the formation of complex 3 from the reaction of the U(III) complex [U(OSi(O^tBu)₃)₄][Kcryptand] and 0.5 equiv. of PPh₃S was not possible. Reactivity studies of the U(IV) sulfide complexes showed that the sulfide is easily transferred to CO₂ and CS₂ to afford S-functionalized products. Moreover, we have found that the sulfide provides a convenient precursor for the synthesis of the corresponding U(IV) hydrosulfide, {[(SH)U(OSi(O^tBu)₃)₄][K18c6]}, 5, after protonation with PyHCl. Finally, DFT calculations were performed to investigate the nature of the U–S bond in complexes 1, 3 and 5. Based on various analyses, triple-bond character was suggested for the U–S bond in complexes 1 and 3, while double-bond character was determined for the U–SH bond in complex 5.