For the first time, we introduce hollandite-type K_0.17TiO₂, in which potassium ions are located at the center of a (2 × 2) tunnel structure, as a potential cathode material for potassium-ion batteries. Density functional theory calculation predicts the possibility of K⁺ insertion into the hollandite-type tunnel structure via a single-phase reaction. Operando X-ray diffraction and X-ray absorption spectroscopy analyses verify that potassium ions are de-/intercalated from/into the crystal structure of K_0.17TiO₂, accompanied by a Ti⁴⁺/Ti³⁺ redox reaction. The single-phase reaction is sustainable for long-term cycling, with exceptionally high operation voltage over 2.5 V. The hollandite-type K_0.17TiO₂ cathode delivers a reversible capacity of 60 mAh g⁻¹ at 5 C (1.55 A g⁻¹), with excellent capacity retention of over 98% of the initial capacity for 1000 cycles. This performance is related to the single-phase reaction with good structural stability. This work presents a facile approach that enables the use of a cathode with stable tunnel structure for potassium-ion batteries.