Red upconversion luminescence (UCL) nanoparticles are of significant importance for applications in the fields of deep tissue imaging, photothermal therapy and security ink. In this work, a highly efficient red emission was achieved by introducing Yb³⁺ ions as mediators in Er³⁺ self-sensitized Gd₂O₂S nanoparticles under excitation at 1530 nm. The results show that the Gd₂O₂S:Yb³⁺,Er³⁺ nanoparticles synthesized by a homogeneous precipitation method exhibit a uniform spherical shape and narrow size distribution with a mean particle diameter of ≈65 nm. Moreover, the integral emission intensity ratio of red to green of the Gd₂O₂S:Yb³⁺,Er³⁺ sample is significantly enhanced 3-fold compared with the Gd₂O₂S:Er³⁺ sample without Yb³⁺ doping. The enhancement mechanisms are discussed in detail on the basis of steady-state luminescence spectra and decay dynamics measurements under various excitations at 380, 808, 980 and 1530 nm, respectively. It has been demonstrated that the enhanced red luminescence is induced by cross-relaxation energy transfer from Er³⁺ to Yb³via⁴S_3/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺) and ⁴I_11/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_15/2 (Er³⁺) + ²F_5/2 (Yb³⁺), and further followed by back energy transfer from Yb³⁺ to Er³⁺ through ⁴I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺) → ⁴F_9/2 (Er³⁺) + ²F_7/2 (Yb³⁺). The former cross-relaxation procedure effectively populates the red emission level of ⁴F_9/2 by depopulating the green emission level of ³S_3/2. Our findings provide a feasible way to enhance the red UCL and new insights into red UCL mechanisms in the Er³⁺ self-sensitized system under ≈1500 nm excitation, by combining with the nontoxic oxysulfide host, indicating their potential application as safe fluorescent nanoprobes in the bio-field.