A novel wideband, triple-resonant sectorial dipole antenna with simple configuration is advanced. The antenna is evolved from a 1.5-wavelength prototype electric dipole, which is backed by a circular metallic planar reflector. The flared angle of the sectorial radiator and the corresponding usable resonant modes for wideband radiation is determined by mode synthesis chart. As numerically and experimentally validated, the flared angles of the principal dipole should be 300°, with its radius approximately equal to 0.23-wavelength centered at 2.4 GHz. By introducing a pair of tuning stubs and a pair of slits along circumference, three resonant modes simultaneously excited in a single sectorial dipole to generate a wideband, triple-resonant radiation characteristic. Prototype antennas are then fabricated to experimentally validate the design approach. It is shown that the proposed antenna exhibits a unidirectional pattern with its impedance bandwidth over 73.3%. Meanwhile, stable radiation patterns with gain fluctuation less than 2.4 dB, low cross polarization level of −27 dB, and maximum gain of 6.9 dBi are realized within the same frequency range.