An electronic device may be provided with a speaker box antenna for transmitting and receiving radio-frequency signals. A speaker box antenna may be formed from a hollow dielectric speaker box containing a speaker driver. An opening in the speaker box adjacent to the speaker driver may be aligned with a speaker port opening in a conductive electronic device housing structure. The speaker box may be surrounded by conductive structures that form a cavity for the antenna. The conductive structures may include parts of the conductive electronic device housing structure. The speaker box may have opposing upper and lower surfaces. Metal plates may form parts of the upper and lower surfaces and may be shorted together using a conductive layer such as a strip of metal tape. Frequencies of operation may be selected for the antenna that suppress undesired cavity modes and enhance antenna performance.