Calcium-ion intercalation-based batteries receive attention as one type of post lithium-ion battery because of their potential advantages in terms of cost and capacity. A birnessite-type manganese oxide, K0.31MnO2·0.25H2O, is characterized by a layered structure with interlayer distances of ∼7 Å. Here, we demonstrate for the first time the electrochemical Ca2+ ion intercalation capability of K-bir, and elucidate the calcium-ion storage mechanism. A reversible electrochemical reaction is observed in cyclic voltammograms and galvanostatic cycles. The initial specific discharge capacity is 153 mAh g−1 at 25.8 mA g−1 (0.1 C) in a 1 M Ca(NO3)2 aqueous electrolyte, with the average discharge voltage of 2.8 V (vs. Ca/Ca2+). X-ray diffraction, transmission electron microscopy, and elemental analyses confirm that Ca2+ ion transport is mainly responsible for the electrochemical reaction. A kinetic analysis using CVs with various scan rates indicates that the reaction mechanism can be described as a combined reaction of a surface-limited capacitance and a diffusion-controlled intercalation. In addition, 3D bond valence sum difference maps show the 2D network for conduction pathways of calcium ions in the structure. This work demonstrates that birnessite-type manganese oxide could be a potential cathode material for calcium-ion batteries.