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A key challenge for potassium‐ion batteries is to explore low‐cost electrode materials that allow fast and reversible insertion of large‐ionic‐size K+. Here, we report an inorganic‐open‐framework anode (KTiOPO4), which achieves a reversible capacity of up to 102 mAh g−1 (307 mAh cm−3), flat voltage plateaus at a safe average potential of 0.82 V (vs. K/K+), a long lifespan of over 200 cycles, and K+‐transport kinetics ≈10 times faster than those of Na‐superionic conductors. Combined experimental analysis and first‐principles calculations reveal a charge storage mechanism involving biphasic and solid solution reactions and a cell volume change (9.5 %) even smaller than that for Li+‐insertion into graphite (≈10 %). KTiOPO4 exhibits quasi‐3D lattice expansion on K+ intercalation, enabling the disintegration of small lattice strain and thus high structural stability. The inorganic open‐frameworks may open a new avenue for exploring low‐cost, stable and fast‐kinetic battery chemistry. [ABSTRACT FROM AUTHOR]