This letter introduces a new K-/Ka -band diplexer-integrated simplified rotary joint. The stator and rotor are arranged to construct a guided-wave structure based on gap waveguide (GW) technology, which supports a stable rotation without leakage. A bandpass filter (lower band channel) and a two-path high-pass filter (upper band channel) are designed on the stator to form a diplexer to enable the wideband dual-band operation. A coaxial probe-based common port is located at the center of the rotor to achieve a smooth signal transmission in both bands. For demonstration, a prototype operating at 17.7–21.2 GHz ( Ka -band) and 27.5–31 GHz ( Ka -band) is designed, fabricated, and measured. Experimental results show that the rotary joint has the maximum insertion loss of 0.4 and 0.9 dB in the two passbands. During the rotation, the insertion loss variation is less than 0.2 dB, guaranteeing a stable performance.