Development of new contactless thermometers with improved characteristics is highly demanded for science and technology. In this work, we present upconverting Tm³⁺/Yb³⁺-doped GeO₂–PbO–PbF₂ glass ceramics as optical temperature sensor. Upon 973 nm laser excitation, sample generates intense emission bands from blue to NIR regions due to ladder-like energy levels of Tm³⁺ ion. A ratiometric technique involving the Tm³⁺ thermally and non-thermally coupled levels (¹G_4(a), ¹G_4(b), ³F_2,3, ³H₄) and spectral line position of ³H₄–³H₆ transition was used to provide thermal sensing in the 300–466 K temperature range. The performance of suggested sensing parameters was compared and discussed in terms of relative thermal sensitivity. The maximal thermal resolution, which can be achieved using Tm³⁺/Yb³⁺-doped GeO₂–PbO–PbF₂ thermometers, is better than 1 K at both 303 and 465 K. The obtained results show the potential use of GeO₂–PbO–PbF₂: Tm³⁺,Yb³⁺ phosphor for optical sub-degree temperature sensing.