In this letter, a new compact filtering power divider (FPD) on the square substrate integrated waveguide (SIW) with wideband isolation is proposed. As the isolation network is embedded in a square SIW cavity, good in-band isolation, compact structure, and low insertion loss can be simultaneously attained. The proposed FPD consists of the coplanar waveguide feeding line of Port 1, the capacitor, a resistor, and the square SIW cavity. TE101, TE201, and slot-line modes can realize third-order filtering response. Center frequency (f0) and bandwidth of the filtering response can be independently adjusted. In order to achieve out-of-band isolation, Port 2/3 is placed in the specific position to suppress the high-order modes according to the electric-field distributions of these modes. Finally, a prototype with in-band return loss of 20 dB across 5.59-6.4 GHz is designed at 5.99 GHz. The bandwidths of isolation higher than 21 dB and upper stopband attenuation higher than 20 dB are extended to 2.79f0 and 2.83f0, respectively.