Ether-based room-temperature sodium–sulfur (RT Na
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S) batteries are a promising energy-storage system, yet hindered by the unregulated sulfur redox pathway, severe polysulfide shuttling and rapid capacity fading. Herein, highly unsaturated niobium-oxide sub-nanoclusters (≈0.7 nm) anchored on defective carbon black (NbOx-DCB) as a dynamic sulfur-conversion catalyst are introduced. The delocalized Nb d-electrons in the sub-nanocluster configuration create a mixed Nb4+/Nb5+ valence state that functions as a bidirectional electron reservoir, thereby enabling a distinct d-band-center self-regulation mechanism. The strong d–p orbital coupling enabled by a Nb4+-rich surface effectively captures sodium polysulfides and accelerates sulfur conversion kinetics during discharge, while a Nb5+-rich surface promotes facile solid-polysulfide decomposition during charging. Consequently, the NbOx-DCB/S cathode delivers a reversible capacity of 1184 mAh gS−1 at 0.1 A g−1 after 100 cycles and retains 547 mAh gS−1 after 3000 cycles at 2 A g−1, corresponding to a decay rate of 0.0027% per cycle. The general applicability of this approach is validated by high-performance tungsten and vanadium oxide sub-nanocluster-based sulfur cathodes. These findings highlight sub-nanoscale metal-oxide engineering as a versatile route to high-performance RT Na–S batteries.