Frequency-adaptive bandpass diplexer (FA-BPD) with constant absolute bandwidths (ABWs) and two identical passbands at every state is the most useful duplexing tunable filter for the smart frequency-division duplex (FDD) system. However, it is widely considered a challenge. In this article, we propose a new synchronously tuned trimode resonator (STTR) to achieve such a type of FA-BPDs. The proposed STTR has three flexible resonant modes that are able to be synchronously tuned by only one bias with the predefined frequency spacings. The spacings between every two tunable resonant frequencies can be precisely controlled as desired. Consequently, the resultant tunable filters can be implemented with different constant ABWs and at the different center frequency, which makes it possible to achieve two frequency-adaptive bandpass filters (FA-BPFs) at different frequency ranges but with the same ABW. Based on the proposed STTR, a three-pole FA-BPF with nearly 250-MHz ABW is examined. Using the FA-BPF as basic building blocks, a 1.5–3.5-GHz FA-BPD with identical passband and constant ABW is developed finally. The simulation and measurement results confirm the proposed design method and also exhibit the merits, such as low loss, simple design, wide frequency tuning range with synchronous passband.