Electrocatalysis-assisted entrapment of polysulfide while ensuring efficient nucleation of Na2S holds the key to addressing the shuttle effect and sluggish kinetics of polysulfide in room-temperature (RT) Na/S batteries. The constrained active sites, however, dramatically limit the efficiency of electrocatalysts. Here, a strategy of electrochemically releasing nano-silver catalytic sites during the discharge process is presented, visualized, and implemented for accelerated Na2S nucleation. Because of the effective polysulfide immobilization and accelerated Na2S nucleation, the sulfur cathode, supported by a self-released silver electrocatalyst, exhibits a superior reversible capacity of 701 mAh g−1 at 0.1 A g−1 and an ultra-stable cycling performance. Precise understanding of the electrochemically self-releasing mechanism and the catalysis in Na2S nucleation via in situ transmission electron microscopy (TEM) would aid, however, in fundamentally optimizing the working mechanism and for further development of more stable high-power RT Na/S batteries.