We report on Nyquist wavelength division multiplexing (WDM) communication in the terahertz (THz)-band utilizing an integrated-optic spectrum synthesizer for the generation of a Nyquist WDM signal. The synthesizer comprises two arrayed-waveguide gratings, and an array of variable optical attenuators and phase shifters, and it can individually control amplitude and phases of up to 64 frequency components with 40 GHz spacing. Three optical frequency combs were produced with the combination of a mode-locked laser diode and the synthesizer for the THz-wave Nyquist WDM signal generation. We demonstrate 2 x 40 Gbit/s Nyquist WDM communication in the 300 GHz band.