One-dimensional (1D) single-crystalline trigonal selenium nanorods (t-Se NRs) were prepared through a “solid–solution–solid” method by dispersing the prepared amorphous α-Se spheres in ethanol. The single crystal structure of the t-Se NRs could be excited under visible light, with a band gap of 1.56 eV. The t-Se NRs were found for the first time to display noticeable photocatalytic activity under visible light. In addition, the rate of photodegradation of methyl orange (MO) in the visible–Se NRs–H₂O₂ system (1.7 × 10⁻¹ min⁻¹) is about 17 times greater than that in the visible–Se NRs–HCl system with equal pH (1.0 × 10⁻² min⁻¹). Furthermore, we proposed a photocatalysis mechanism of the visible–Se NRs–H₂O₂ system based on electron paramagnetic resonance (EPR) characterization and studying the effect of different reactive species.