Novel mesoporous Li₂MnO₃/g-C₃N₄ heterostructures were prepared for the first time by utilizing the sol–gel route in the presence of a nonionic surfactant. TEM and XRD measurements showed that Li₂MnO₃ (5–10 nm) with monoclinic structures was uniformly distributed onto porous g-C₃N₄ for the construction of Li₂MnO₃/g-C₃N₄ heterojunctions. The obtained photocatalysts were assessed for mineralization and removal of trichloroethylene (TCE) in aqueous media under visible light exposure. Complete degradation of TCE over a 3 %Li₂MnO₃/g-C₃N₄ heterostructure within 120 min was achieved. The degradation rate over Li₂MnO₃/g-C₃N₄ heterostructures was significantly enhanced, and the 3% Li₂MnO₃/g-C₃N₄ heterostructure exhibited a large degradation rate of 7.04 µmolL⁻¹ min⁻¹, which was enhanced by 5 and 3.8 fold compared to those of pristine g-C₃N₄ (1.39 µmolL⁻¹ min⁻¹) and Li₂MnO₃ (1.85 µmolL⁻¹ min⁻¹), respectively. The photocatalytic efficiency of the Li₂MnO₃/g-C₃N₄ heterojunction was outstandingly promoted because integrating Li₂MnO₃ with g-C₃N₄ could create close interfaces with well-matched band potentials for easy mobility and low recombination of photoinduced carriers. The coexistence of Li₂MnO₃/g-C₃N₄ interfaces led to a synergic effect, which is considered the key factor in photoinduced electron-hole separation. The synthesis procedure that was employed here is a promising process for the preparation of effective g-C₃N₄-based photocatalyst systems for photocatalysis applications.