Here, we report Nd³⁺-doped Y₂O₃ nanoparticles suitable for luminescence thermal sensing in the first and second biological windows. The nanoparticles were synthesized via the combined Pechini-foaming method. A ratiometric approach, based on the relative changes in the intensities of different emission bands corresponding to the Stark sublevels or excited levels, was applied to determine local temperature. The evaluated thermal sensitivities differed 5-fold times depending on the choice of transitions for the luminescence intensity ratio calculation. The temperature uncertainty was determined to be below 1 °C, which allows to perform sub-degree thermal sensing. The results of ex vivo experiment indicate that Nd³⁺-doped Y₂O₃ nanoparticles are promising candidates for real biological applications.