A fluid switch is proposed for a frequency-agile Vivaldi antenna whose operating frequency band can be switched between two selected bands. A study of various ionized solutions of different concentrations is performed. A 2 mol KCl solution is selected as the fluid for the switch because of its relatively good properties in conductivity, relative permittivity, and loss tangent. The fluid-switched reconfigurable Vivaldi antenna can function well between two user-defined operating bands: 3.2 and 4.5 GHz with stable measured gain of 11 dBi in both bands and an isolation of 15 dB. This reconfigurable antenna demonstrates that a low-cost fluid switch may be an alternative device for reconfigurable antenna designs providing more flexibility.