Rare earth doped phosphors have been extensively used as self-calibrated ratiometric thermometers for their contactless, noninvasive and high precision properties. However, the drawbacks of low relative sensitivity and signal discernability in conventional thermally coupled energy level (TCL)-based temperature sensing need to be circumvented. Herein, we report a novel ratiometric optical thermometer, a Dy³⁺, Eu³⁺ co-doped GdAl₃(BO₃)₄ phosphor, based on the significantly different thermal sensitivities of emissions from Dy³⁺ and Eu³⁺. The micron-sized phosphor in the pure hexagonal phase was prepared via a high-temperature solid state method. Dy³⁺ and Eu³⁺ can be well sensitized by Gd³⁺ in the matrix under 311 nm excitation and produce well-separated emission bands. The energy transfer from Dy³⁺ to Eu³⁺ was verified by the photoluminescence spectra and decay curves, and it greatly alleviated the thermal quenching of emission from Eu³⁺. By using the emission of Eu³⁺ at 613 nm as the reference signal and Dy³⁺ at 574 nm as the detection signal, ratiometric optical thermometry was realized at 300–475 K. The maximum relative sensitivity is 1.37% K⁻¹ at 475 K. It is superior to those of the Dy³⁺ single-doped GdAl₃(BO₃)₄ phosphor (0.71% K⁻¹ at 400 K) and many other recently reported phosphors based on the TCLs. This work provides a new candidate optical thermometer with superior relative sensitivity and good signal discriminability.