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This work aimed to explore the temperature-sensing performance of La₂MgTiO₆:Er³⁺ double perovskites based on thermally coupled and uncoupled energy levels. Furthermore, the crystal structure, chemical composition, and morphology of the samples were investigated by powder X-ray diffraction, energy-dispersive X-ray spectroscopy, and scanning electron microscopy, respectively. The most intense luminescence was observed for the sample doped with 5% Er³⁺. The temperature-dependent emission spectra of La₂MgTiO₆:5% Er³⁺ were investigated in the wide range of 77–398 K. The highest sensitivity of the sample was equal to 2.98%/K corresponding to the thermally coupled energy level ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 as compared to 1.9%/K, obtained for the uncoupled energy level ²H_11/2 → ⁴I_15/2 and ²H_9/2 → ⁴I_15/2. Furthermore, the 300 K luminescent decay profiles were analyzed using the Inokuti–Hirayama model. The energy transfer among Er³⁺ ions was mainly regulated by the dipole–dipole mechanism. The critical transfer distance R₀, critical concentration C₀, energy transfer parameter C_da, and energy transfer probability W_da were 9.81 Å, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>2.53</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>20</mn></mrow></msup></mrow></semantics></math></inline-formula> ions·cm⁻³, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>5.38</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>39</mn></mrow></msup><mo> </mo></mrow></semantics></math></inline-formula>cm⁶·s⁻¹, and 6020 s⁻¹, respectively.