A beamwidth reconfigurable linear array antenna utilizing three magneto-electric (ME) dipole antenna elements with no need for a complex feeding network is presented. In this design, the ME dipole antenna elements are differentially excited by three parallel transverse resonant slots etched on the broad wall of a substrate integrated waveguide (SIW) cavity. By symmetrically embedding four copper posts that are connected to varactor diodes between the adjacent slots, the electric field perturbation in the SIW cavity can be realized by manipulation of the varactors. In other words, altering bias voltage of all the varactors simultaneously could offer granular control over the excitation amplitude and phase of the antenna elements. Consequently, continuously variable beamwidth of the antenna can be obtained under electronic control. A fully operational prototype was designed, fabricated, and tested to validate the proposed design. Measured results indicate that the overlapped -10 dB impedance bandwidth of the proposed antenna is 19.6% and the half-power beamwidth (HPBW) in the E-plane is capable of varying from 39° to 107° within the operating frequency band.