Luminescence thermometry has attracted a lot of attention due to its ability to provide temperature sensing with high spatial, temporal, and thermal resolution. This technique is the most promising for probing changes in local temperature in living cells to monitor thermogenesis and to study cellular activity. Despite rapid progress in this field, there is still a lack of optical thermometers capable of thermal sensing at the subcellular level, especially owning additional functionality. Here, cationic (meso-mono(4-pyridyl)-triphenylporphyrinato)phosphorus(V) bromide MPyPP(OH) ₂ was used as subcellular ratiometric thermal sensor in CHO-K1 and HeLa cells within the biological temperature range (30–44 ^oC). Introduction of porphyrin into cells resulted in its dephosphorylation and change of luminescence properties. Ratiometric sensing was performed using peak-to-peak and peak-to-valley ratios. Thermometric performance was evaluated in terms of absolute and relative sensitivities, temperature resolution. Relative thermal sensitivity and temperature resolution reached 0.61% K ^-1 and 0.1 ^oC, respectively in the case of CHO-K1 cells.