The reactions of the tetrasiloxide U(III) complexes [U(OSi(OtBu)3)4K] and [U(OSi(OtBu)3)4][K18c6] with 0.5 equiv. of triphenylphosphine sulfide led to reductive S-transfer reactions, affording the U(IV) sulfide complexes [SU(OSi(OtBu)3)4K2]2, 1, and [{SU(OSi(OtBu)3)4K2}2(μ-18c6)], 2, respectively, with concomitant formation of the U(IV) complex [U(OSi(OtBu)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(OtBu)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(OtBu)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(OtBu)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 complex 1 and 3, while double-bond character was determined for the U-SH bond in complex 5.