A terahertz integrated amplifier circuit packaged inside a WR-3.4 waveguide is presented. The circuit, fabricated using 250-nm InP technology, contains three amplifier chains each connected to its own input and output waveguide transition pairs. The size of the transition is minimised for the chip to properly fit inside the standard waveguide. The assembled waveguide module shows the peak amplifier gain of 13 dB and 3-dB bandwidth of 32 GHz as its gain is 6.2 dB less than that of an on-wafer amplifier measured using RF probes. The maximum measured output power from the module is 6.5 dBm at 260 GHz compared to 1.4 dBm from single on-wafer amplifier chain.