A new high-order wideband planar balun filter is proposed in this paper. The balun filter is exact synthesized based on S-domain bandpass prototype using Richards' Theorem to fit the user-defined specifications. By analyzing the redundant element in the equivalent circuit, the output impedance of the balun filter can be arbitrarily assigned. To use an S-plane bandpass prototype other than a high-pass prototype can shrink the circuit area and improve the stopband performance. The lumped capacitor approximation suppresses the spurious response in the stopband and further reduces the circuit size. A high-order balun filter with a bandwidth of 100% and a reflection coefficient of -15 dB is implemented by combining of microstrip lines, slotlines, and coplanar striplines. The simulation and measurement results match well.