In this letter, a 263 GHz traveling wave tube for electron paramagnetic resonance spectroscopy is designed, fabricated and tested. A periodic permanent magnet focused pencil beam electron optical system is adopted. A folded waveguide slow-wave structure with modified serpentine bends is optimized to provide high-power wideband performance and stable operation. An experiment has been performed to verify the analysis results and confirm the amplifier stability. The device provides a maximum 11.9 W saturation output power and 25.5 dB saturation gain. Although the available solid-state signal source is unable to drive the amplifier to saturation beyond 260 - 264 GHz, 10 W output power over 5.6 GHz bandwidth has been measured.