[en] Highlights: • A heavily Yb³⁺-substituted langbeinite-type single crystal has been successfully grown by high temperature solution method. • Dual-color upconversion photoluminescence was observed in the highly stable Rb₂Ti_0.80Yb_1.20(PO₄)₃ single crystal. • Disordered Yb³⁺-Ti⁴⁺ pairs and crystal defect were confirmed by X-ray structural analyses. • Blue and red emissions are ascribed to the cooperative luminescence and intrinsic defect. -- Abstract: Inorganic upconversion materials show significant importance in both basic research and applied area, and searching for suitable host material and understanding of fundamental photophysical process are particularly worth studying in upconversion application. Herein, a heavily Yb³⁺-substituted langbeinite-type Rb₂Ti_0.80Yb_1.20(PO₄)₃ (RTYP) millimeter-sized single crystal, which has been successfully grown by high temperature solution method, exhibits efficient dual-color upconversion photoluminescence under 980 nm excitation. The lifetimes of two-photon blue emission and one-photon red emission are 4.1 μs and 17.1 μs, respectively. X-ray structural analyses reveal that the dual-color emissions are ascribed to disordered Yb³⁺-Ti⁴⁺ pairs and crystal defect, and the higher intensity of red emission is attributed to Yb³⁺-defect energy transfer activated by Ti⁴⁺. Notably, the blue emission originates from cooperative luminescence of Yb³⁺ dimer, and the red emission originates dominantly from the cooperative sensitization of the intrinsic defect caused by excited Yb³⁺ dimer. Moreover, RTYP with disordered structure shows high thermal stability up to 1,150 °C. This work not only demonstrates the promising potential of langbeinite-type materials in multicolor upconversion application, but also sheds light on the photophysical process associated with disordered crystal structure.