Antimony trisulfide-based materials have drawn growing attention as promising anode candidates for potassium-ion batteries (PIBs) because of their high capacity and good working potential. Despite the extensive investigations on their electrochemical properties, the fundamental reaction mechanisms of Sb 2 S 3 anodes, especially the reaction kinetics, structural changes, and phase evolutions, remain controversial or even largely unknown. Here, using in situ transmission electron microscopy, the entire potassiationdepotassiation cycles of carbon-coated Sb 2 S 3 single-crystal nanowires are tracked in real time at the atomic scale. The potassiation of Sb 2 S 3 involves multistep reactions including intercalation, conversion, and two-step alloying, and the final products are identified as cubic K 2 S and hexagonal K 3 Sb. These findings are confirmed by density functional theory calculations. Interestingly, a rocket-launching-like nanoparticle growth behavior is observed during alloying reactions, which is driven by the K + concentration gradient and release of stress. More impressively, the potassiated products (i.e., K 3 Sb and K 2 S) can transform into the original Sb 2 S 3 phase during depotassiation, indicating a reversible phase transformation process, as distinct from other metal chalcogenide based electrodes. This work reveals the detailed potassiation/depotassiation mechanisms of Sb 2 S 3-based anodes and can facilitate the analysis of the mechanisms of other metal chalcogenide anodes in PIBs.