Sodium-ion batteries (SIBs) are recognized as a promising alternative for lithium-ion batteries (LIBs) in large-scale energy storage applications, because of the low cost and abundant sodium resources. Electrode materials govern the electrochemical performance of SIBs and are crucial to their development. Sodium manganese oxide (NaxMnO2) is widely studied as cathode materials of SIBs, because of its structural diversity and rich manganese resources. It exhibits many polymorphs and different structural characteristics with the change of sodium contents, including layered, three-dimensional tunnel, and spinel structures. Herein, a comprehensive review on the structure features, charge compensation, phase transition, and the relationship between the structural characteristics and electrochemical performance of sodium manganese oxide as cathode materials of SIBs is presented, and a perspective is provided for the development of SIBs.