In this letter, an independently 1.1–2.8 GHz tunable four-channel cavity diplexer is proposed and developed. The presented diplexer consists of two tunable dual-band filters. Each filter is separately achieved by putting two identical sets of two metal posts in a single cavity. The variable distance of the posts is selected to provide an appropriate coupling between adjacent modes during the tuning process. Meanwhile, the adjustable screws with capacitive loading are employed to control the prescribed resonant frequency of the cavity. Thus, a wide tuning range of 1.1–2.8 GHz (87% tunable ratio) is obtained in a continuous way. Moreover, the diplexer can achieve an isolation higher than 50 dB. Finally, the proposed diplexer is fabricated, and the measured results agree well with the simulated results.