This letter presents a novel millimetre-wave (mm-wave) on-chip substrate integrated waveguide (SIW) filtering crossover using 0.25 µm GaAs pHEMT technology. The design methodology of the proposed crossover is thoroughly illustrated. The proposed filtering crossover employs a dual-mode cavity with TE and TE degenerate mode resonances at the intersection, leveraging the degenerate modes for in-band resonance and inter-channel isolation. Additionally, four TE mode resonant half-mode SIW cavities are coupled around the dual-mode cavity to achieve two third-order bandpass response channels and reduce the overall size. A prototype is designed, analysed, and fabricated to validate the proposed approach, with measured results showing good agreement with simulations. The presented on-chip SIW filtering crossover offers promising potential for mm-wave applications, demonstrating the effectiveness of the design methodology and GaAs pHEMT technology integration.