This brief proposes a wideband millimeter-wave (mm-wave) single-pole single-throw (SPST) switch using strong mutual coupling. For on-chip switches, it is very challenging to realize wide bandpass characteristics, especially in mm-wave bands. To address this issue, a wideband switch topology using strong mutual coupling has been introduced, based on which the fractional bandwidth of SPST switch can be easily expanded to over 80%. In addition, the coupling structure using coupled lines is investigated to realize strong mutual coupling. For demonstration, one 20-44.8 GHz switch with two transmission poles (TPs) is designed and fabricated in 100-nm GaN-on-Si. Low ON-state minimum insertion loss of 0.62 dB, preferable return losses better than −20 dB, and small chip size of 0.165 are realized, respectively.