K_0.5Na_0.5NbO₃ (KNN)-based ceramics, with high Curie temperatures and good ferroelectric performance, show promising potential as functional materials. In this study, the incorporation of CaTiO₃ (CT) into KNN-based ceramics resulted in a high density, submicrometer grain size, and pseudocubic phase, which collectively contributed to an impressive infrared transmittance of 78% at 1336 nm, as well as enhanced relaxor ferroelectric properties. A slim P–E loop and well-defined polar nanoregions (PNRs) can be clearly observed in the samples when the CT content exceeds 0.175. Moreover, we demonstrate that ultrahigh sensitivity in temperature sensing can be achieved via Pr³⁺-doped KNN–CT on the basis of the fluorescence intensity ratio (FIR) of the ¹D₂–³H₄ emission to the ³P₀–³H₅ emission. The FIR increases by as much as 75-fold when the temperature increases from room temperature to 500 K, resulting in exceptional absolute sensitivity (S_a = 0.139 K⁻¹) and relative sensitivity (S_r = 2.69 %/K). This exceptional FIR performance is believed to be closely linked to the temperature-sensitive PNRs in KNN–CT, highlighting its significant potential for applications in optical temperature sensors.