In this paper, a new design of a highly isolated tri-band antenna for 5G and future 6G applications is proposed. The overall dimensions of the proposed antenna are just 56.4 × 36.6 mm 2 ; moreover, it contains two monopole antenna units and a defective ground. The tri-band characteristics of the antenna are achieved by improving the single antenna patch structure. The structure is improved based on the original T-shaped decoupling structure to create a fence-shaped decoupling structure consisting of an improved T-shaped and an improved rectangular structure. This will greatly improve isolation by efficiently absorbing the coupling current. Therefore, the proposed antenna system is designed and tested to reach the 5G dual bands of 3.38 GHz-3.61 GHz, 4.51 GHz-4.96 GHz, and the future 6G band of 6.06 GHz-7.51 GHz. The results show that, relative to other antennas, this antenna has an isolation degree in the operating band greater than 13.1 dB. In addition, the antenna has good radiation characteristics and an acceptable envelope correlation coefficient.