In this paper, a novel microstrip diplexer with tunable bandwidths and switchable channels is presented by using dual-mode square loop resonators (DMSLRs). Two resonators at different electrical lengths are coupled to input and output ports with lumped capacitors, and the isolation between the output ports is optimized by means of these capacitors. In order to obtain tunable bandwidth in each channel, varactor diodes are located at the orthogonal corners of the proposed resonators. In addition, the proposed resonators have reference patch elements at the lateral arms to provide convenience of varactor diode capacitances, such as perturbation elements. Depending on the capacitances of the varactor diodes, the bandwidths of each channel can be tuned. Furthermore, each channel can be switched off. The designed diplexer has two channels at the center frequencies of 1.8 and 2.6 GHz, which are allocated to 4.5G applications. Three-dB bandwidths can be tuned between 60 and 160 MHz for the first channel and between 150 and 350 MHz for the second channel. Isolation between the output ports is obtained as better than 18 dB in each channel. The designed diplexer was fabricated and measured, and the measured results were consistent with the predicted results.