This study focuses on the design of an antenna for smartwatches that is suitable for use in the fifth-generation (5G) communication system. The antenna design utilizes the radiation characteristics of slot antennas, with a single feed as the design basis, to develop the antenna within limited space. Based on the fabricated prototype and actual measurements, the proposed design can be operated in the n78 frequency band (3.3 GHz to 3.8 GHz) of the 5G system, as well as in the Wi-Fi 6E frequency band (5.925 GHz to 7.125 GHz). Furthermore, the design incorporates the discontinuous surface generated by the peripheral annular slots, allowing for charge redistribution and radiation, and the irregular structure of the ground plane can excite additional resonance points, which features satisfy the requirements of the miniaturization, simplicity and single-feed for the multi-band antenna design.