We demonstrate a decade-passband superconducting diplexer operating from dc to 27 GHz, integrated in a niobium, Josephson junction (JJ) compatible process. Both low- and high-pass branches of the diplexer are singly-terminated 5-pole Butterworth filters with a 2.5 GHz cutoff. Several diplexer test circuits were characterized with on-wafer cryogenic two-port calibration by terminating the third port of the device. Insertion loss below 0.8 dB are measured across the entire frequency range. Good agreement between measurements and simulations up to 27 GHz suggests that the upper diplexer band extends to the over-a-decade simulated 40 GHz. This device can be integrated in large-scale superconducting JJ-based quantum processors, broadband sensors, amplifiers and sources.