Abstract: A novel four-port filter topology is presented for obtaining additional transmission zero whereas remaining the reflectionless characteristics. By introducing additional capacitor/inductors in the shunt branches of the original filter topology, an extra transmission zero is observed in the stopband closed to the cut-off frequency, which is helpful for sharp rejection. Meanwhile, the extra zero contributes flatter in-band transmission due to the complementary nature of the four-port architecture. The ideal values of all elements are obtained after theoretic derivation, and then verified by using circuit models. Comparing with the traditional four-port reflectionless filters, the simulated results show that this filter has tunable out-of-band attenuation curve with more flexible shapes by adjusting the location of the transmission zeros. Particularly, it can be tuned to have an equiripple response in both passband and stopband. A lowpass/highpass filter and a bandpass/bandstop filter are designed and fabricated for verification. The experimental data agrees well with the prediction.