A novel wideband out-of-phase power divider based on Klopfenstein tapered line is proposed for the first time. By using double-sided parallel-strip line (DSPSL) with inserted conductor plane as virtual ground, frequency-independent phase difference can be achieved with antisymmetric output structures. Odd-/even-mode analysis shows that the introduced Klopfenstein tapered line can expand bandwidth greatly, and additional eight pairs of isolation resistors are added to realize high output matching and isolation. For verification, this presented power divider is fabricated and measured. The simulation and measurement show that this structure can achieve a wide frequency range from 0.5 GHz to 7.3 GHz, within which the S-parameters (S11, S22, S32) are all better than −10 dB, and the phase imbalance is less than 7°.