An electronic device may have polarizer layers, color filter layers, thin-film-transistor layers, and other display layers. A display layer may be separated from structures such as a display cover layer formed from clear glass or plastic and a touch sensor layer mounted on an inner surface of the display cover layer by an air gap. Cavities within an electronic device housing may serve as a source of dust and other contaminants. The air gap may be sealed against dust intrusion from a cavity within an electronic device using a dust sealing structure. The dust sealing structure may have a rectangular ring shape that runs around a rectangular peripheral portion of a display layer. The dust sealing structure may be formed from a tape-based structure, an elastomeric structure, a compressible foam structure, or a cured liquid structure.