We demonstrate a novel type of traveling wave photodetector: "metal-semiconductor-metal traveling-wave photodetector" (MSM-TWPD). Demonstrated devices were fabricated using low-temperature grown GaAs (LTG-GaAs). In order to achieve high internal quantum efficiency, the narrow spacing between electrodes was fabricated by the self-aligned process without e-beam lithography. Electrooptical sampling measurement results at different optical pumping level are reported. Ultrahigh bandwidth (0.8-ps, 570-GHz transform bandwidth) performance was observed even under high optical power illumination (∼1.8 mW) with 8.1% net quantum efficiency. Compared with LTG-GaAs-based p-i-n TWPD and vertically illuminated MSM photodetector (PD), this novel TWPD has higher output saturation current with near terahertz electrical bandwidth, better quantum efficiency, and can be easily fabricated and integrated with other microwave devices. It thus promises the application in high-power distributed PD array or terahertz signal generation.