Abstract: A compact tri-band single-order integrated substrate gap waveguide (ISGW) cavity filter is proposed in this paper. To restrict the mode numbers in the cavity, the ISGW cavity is designed by analyzing the resonating frequency relationship between these modes, which gets three resonant modes in it. To improve the out-of-band suppression and frequency selectivity, a novel tri-band single-order filtering response coupling topology is proposed. Furthermore, on the basis of studying the coupling relatonship between the four ports of the cavity membrane, the position relationship of the input and output ports is designed, which is different from the traditional input and output ports. Each of the input and output ports has a "U" groove feeding structure with a position relationship of 90°. Finally, the proposed tri-band single-order filter that only has a resonating mode in-band is designed. The filter is modeled, simulated and configurated, and then its reflection and transmission coefficients is measured by using the network ana-lyzer. The measured results show that the three center frequencies of three bands are f01 = 24. 25 GHz, f02 = 27. 57 GHz and f03 =31. 14 GHz respectively, and the insertion loss (IL) of three passbands are IL1 = 1. 58 dB, IL2 = 1. 07 dB and IL3 = 2. 51 dB re-spectively. The finite transmission zeros (FTZ) are FTZ1 = 20. 55 GHz, FTZ2 = 26. 20 GHz, FTZ3 = 29. 37 GHz and FTZ4 =33. 17 GHz respectively. The out-of-band suppression between bands is better than 13 dB. There is a certain frequency shift be-tween the measured results and the simulation results, but the fractional bandwidth is better than the simulation results. Therefore, compared with traditional filters, this filter has the advantages of a high design frequency band, high out-of-band suppression level in the millimeter-wave frequency band, strong frequency selectivity, small overall size, and light weight.