This article reviews the structure, synthesis, host-guest reaction in aqueous phase, and ion-sieve and molecular-sieve properties of porous manganese oxide crystals. Tunnel and layered manganese oxides constitute a large family of porous materials having pore size from ultramicropores to mesopores. The manganese oxide crystals consist of MnO ₆ octahedral units shared by corners and/or edges. They can be prepared by using various metal ions or organic surfactants as templates. The templates are extracted/inserted topotactically from/into the tunnels or interlayer spaces of the manganese oxides by two different mechanisms: redox-type and ion-exchange-type. These manganese oxides show excellent ion-sieve and molecular-sieve properties for the adsorptions of cations or organic molecules. The adsorptive selectivities are dependent on their structures.