A novel wide-band traveling wave tube (TWT) based on the double-corrugated waveguide (DCW) is proposed for operating in IEEE G-band (110–300 GHz). The DCW has been conceived for relaxing critical technological issues in terahertz vacuum tube realization, such as assembly, and to support a cylindrical electron beam. The study of the properties of the DCW in the forward wave regime demonstrates wide band performance suitable for TWTs fabrication at millimeter wave–terahertz frequencies. A traveling wave tube with 18-dB gain and up to 3.7 W output power over a bandwidth of about 30 GHz at 225 GHz as central frequency is demonstrated, assuming 13-kV beam voltage and 30-mA beam current.