Micromechanical switches have demonstrated great potential at microwave frequencies. For low-loss applications at microwave frequencies, it is important to use high-resistivity substrates. This paper presents the design and fabrication of the shunt-capacitive MEMS switch on GaAs substrates. Analytical mechanical and impedance models of the membrane switch are given, and the results are confirmed by using the ANSYS and HFSS software, respectively. A surface micromachining process, which is compatible with the conventional millimeter-wave integrated circuits (MMICs) fabrication technology, was adopted to fabricate the RF switch on GaAs substrates. Its S-parameter was taken using a HP8510C vector network analyzer and a Cascade Probe station. The measured insertion loss of the switch and its associated transmission line is less than 0.25 dB from 1 to 25.6 GHz, and the isolation may reach -42 dB at its self-resonate frequency of 24.5 GHz. The actuation voltage is about 17 V. The switch has demonstrated lifetimes as long as 5×106 cycles. The wideband high performance in isolation and insertion loss offers the monolithic integration capability with GaAs MMICs.