Whereas efficient and sensitive nanoheaters and nanothermometers are demanding tools in modern bio and nanomedicine, joining both features in a single nanoparticle still remains a real challenge, despite the recent progresses achieved, most of them within the last year. Here we demonstrate a successful realization of this challenge. The heating is magnetically induced, the temperature readout is optical and the ratiometric thermometric probes are dual-emissive Eu3+/Tb3+ lanthanide complexes. The low thermometer heat capacitance (0.021•K-11) and heater/thermometer resistance (1 K.W-1), the high temperature sensitivity (5.8%K-1 at 296 K) and uncertainty (0.5 K), the physiological working temperature range (295-315 K), the readout reproducibility (>99.5%), and the fast time response (0.250 s), make the heater/thermometer nanoplatform proposed here unique. Cells were incubated with the nanoparticles and fluorescence microscopy permits the mapping of the intracellular local temperature using the pixel-by-pixel ratio of the Eu3+/Tb3+ intensities. Time-resolved thermometry under an AC magnetic field evidences the failure of using macroscopic thermal parameters to describe heat diffusion at the nanoscale.