We have developed radio frequency microelectromechanical systems (RF MEMS) capacitive switches using amorphous diamond (a-D) as a novel tunable dielectric with controlled leakage. The switch is fabricated from sputtered and electroplated metals using surface micromachining techniques. The mechanical stress and resistivity of the a-D dielectric are controlled by the parameters of a high-temperature annealing process. These initial devices exhibit a down-state capacitance of 2.6 pF, giving an isolation of better than 18 dB at 18 GHz, and a predicted static power dissipation of 10 nW. This technology is promising for the development of reliable, low power RF MEMS switches.