A method and apparatus for encoding three-dimensional (3D) image data and a method and apparatus for decoding 3D image data. The method of encoding 3D image data by using a data unit that includes a header area and a data area, wherein type information indicating a type of data included in the data area is included in the header area, includes: encoding reference view image data and additional view image data for providing a 3D image; generating a first data unit by adding to the reference view image data a header including first unit type information from among pieces of type information defined in a legacy device; and generates a second data unit by adding to the additional view image data a header including second unit type information from among pieces of type information not defined in the legacy device.