A class of frequency-reconfigurable reflectionless microstrip bandpass filters is reported. This RF/microwave device is based on a complementary-duplexer architecture made up of a bandpass-type and a bandstop-type channel with resistive termination. Whereas the bandpass channel defines the transmission profile of the overall filter, the input-signal energy that is not transmitted by this channel is absorbed by the bandstop branch. In this manner, a reflectionless behavior at the filter input is achieved. The theoretical foundations of the devised input-reflectionless bandpass filter and synthesis examples for single/multistage designs are presented. Furthermore, for experimental-validation purposes, a two-stage prototype that can be tuned within the 0.8–1.1-GHz range is manufactured and tested.