An image encoding/decoding method and apparatus are provided. The image decoding method comprises obtaining, from a bitstream, video coding layer (VCL) network abstraction layer (NAL) unit type information of a current picture and first flag information specifying whether a subpicture included in the current picture is treated as one picture, determining a NAL unit type of each of a plurality of slices included in the current picture, based on the obtained VCL NAL unit type information, and decoding the plurality of slices based on the determined NAL unit type and the first flag information. The current picture may comprise two or more subpictures based on at least some of the plurality of slices having different NAL unit types, and the first flag information may have a predetermined value specifying that each of the two or more subpictures is treated as one picture based on at least some of the plurality of slices having different NAL unit types.