The massively parallel arrays of highly periodic Gd-doped Si nanowires (SiNWs) self-organized on Si(110)-16 × 2 surface were investigated by scanning tunneling microscopy and spectroscopy. These periodic Gd-doped SiNWs are atomically precise and show equal size, periodic positions, and high-integration densities. Surprisingly, the scanning tunneling spectroscopy results show that each metallic-like, Gd-doped SiNW exhibits room-temperature negative differential resistance (RT-NDR) behavior, which can be reproducible with various Gd dopings and is independent of the tips. Such massively parallel arrays of highly ordered and atomically identical Gd-doped SiNWs with one-dimensional laterally confined RT-NDR can be exploited in Si-based RT-NDR nanodevices.