The dynamic control of the beamwidth of a partially reflective surface (PRS) antenna is reported for the first time. First, a design methodology is proposed by revisiting the PRS antenna theory from the perspective of its reconfigurable beamwidth operation, which is achieved by the control of the reflection magnitude of the PRS. The proposed PRS design only requires one varactor diode per PRS unit cell and a common control voltage for the entire structure. Finally, a fully operational reconfigurable antenna has been fabricated and measured, successfully demonstrating the targeted capability.