This paper presents a multiband antenna implemented on mobile terminals' simple and planar quasi‐self‐complementary (QSC) structure. A wideband antenna is initially developed by employing a triangular patch, and its complementary triangular slot etched on the ground. Then the ground‐cooperative radiating method is combined with the QSC structure. This combination helps to introduce more resonant current paths without increasing the footprint, resulting in additional coverage of lower and higher working bands. The design scheme is validated by both simulation and measurement. Measured results indicate that the proposed antenna provides three separate impedance bandwidths of 310 MHz (0.69–1 GHz), 1070 MHz (1.71–2.78 GHz), and 800 MHz (3–3.8 GHz), covering the desired 5G/LTE/WWAN operations. Meanwhile, moderate antenna efficiencies are obtained across the operating bands. The QSC structure ensures stable frequency responses and acceptable radiation properties for handheld applications. The antenna could be a promising candidate in practical wireless communications due to its desirable performance and slim configuration.