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Sub‐nanoclusters with ultra‐small particle sizes are particularly significant to create advanced energy storage materials. Herein, Sn sub‐nanoclusters encapsulated in nitrogen‐doped multichannel carbon matrix (denoted as Sn‐SCs@MCNF) are designed by a facile and controllable route as flexible anode for high‐performance potassium ion batteries (PIBs). The uniformly dispersed Sn sub‐nanoclusters in multichannel carbon matrix can be precisely identified, which ensure us to clarify the size influence on the electrochemical performance. The sub‐nanoscale effect of Sn‐SCs@MCNF restrains electrode pulverization and enhances the K+ diffusion kinetics, leading to the superior cycling stability and rate performance. As freestanding anode in PIBs, Sn‐SCs@MCNF manifests superior K+ storage properties, such as exceptional cycling stability (around 331 mAh g−1 after 150 cycles at 100 mA g−1) and rate capability. Especially, the Sn‐SCs@MCNF||KFe[Fe(CN)6] full cell demonstrates impressive reversible capacity of around 167 mAh g−1 at 0.4 A g−1 even after 200 cycles. Theoretical calculations clarify that the ultrafine Sn sub‐nanoclusters are beneficial for electron transfer and contribute to the lower energy barriers of the intermediates, thereby resulting in promising electrochemical performance. Comprehensive investigation for the intrinsic K+ storage process of Sn‐SCs@MCNF is revealed by in situ analysis. This work provides vital guidance to design sub‐nanoscale functional materials for high‐performance energy‐storage devices. [ABSTRACT FROM AUTHOR]