An electronic device may be provided with slot antennas. A slot antenna may be formed from metal structures that have a dielectric gap defining an antenna slot. The metal structures may include multiple metal layers that overlap a plastic antenna window and that serve as capacitive electrodes in a capacitive proximity sensor. The metal structures may also include a metal electronic device housing. The metal electronic device housing and the metal layers may be formed on opposing sides of the antenna slot. The metal layers may have a notch that locally widens the antenna slot at an open end of the antenna slot. One of the metal layers may be shorted to the metal electronic device housing at an opposing closed end of the antenna slot. The antenna slot may be indirectly fed using a near-field-coupled antenna feed structure such as a metal patch that overlaps the antenna slot.