Passband flatness and band-edge selectivity in microwave filters with finite quality-factor resonators can be improved by the synthesis of lossy filters. This paper demonstrates the extension of this technique to a lossy diplexer by means of resistive coupling. A dual-mode stub-loaded resonator (SLR) junction and a fork-like feedline are used in the diplexer to address the challenge of independently controlling the external coupling from the common port to the two channel filters and therefore enable flexible realization of the channel bandwidth. The coupling matrices with resistive couplings for the lossy diplexer are generated. For verification, a microstrip lossy diplexer operating at 1.91 and 2.6 GHz was designed and tested. The flatness of the passband has been significantly improved, with a reduction of the passband insertion loss variation from 1.4/1.2 dB to 0.66/0.63 dB for the low/high band. The measured results are in good agreement with the simulations as well as the theoretical responses from the coupling matrix. This was also experimentally compared with a reference diplexer without resistive couplings.