Monodispersed upconversion nanoparticles NaYF4:Er3+/Yb3+ (UCNPs) have been synthesized via the thermolysis method. Förster or fluorescence resonance energy transfer (FRET) dynamics from NaYF4:Er3+/Yb3+ (UCNPs) to gold nanoparticles (AuNPs) are well understood. However, only FRET-based heating (negligible heating from fluorophore or gold) is not much known so far in such systems under near-infrared (NIR) photon excitation. NIR-to-visible upconversion (UC) has already proven its utility in bioassays. Ultrasensitive FRET responsible surface plasmon resonance (SPR) induced hyperthermia has been estimated through water-dispersible monodispersed hybrid UCNP@SiO2@AuNPs via 980 nm NIR laser excitation. The trivial amount of heating for UCNPs and AuNPs are tested under continuous wave (CW) laser excitation. At ∼600 mW laser input power, a temperature increase to ∼43 °C has been noted within 15 min upon CW 980 nm laser excitation. The time-dependent magnetic field study performed with Fe3O4 and hybrid UCNP@SiO2@AuNPs exhibits the usefulness toward the magnetic as well as surface plasmon resonance-based hyperthermia treatment. This hybrid has potential for use as a radio sensitizer in γ-ray irradiation because of the presence of AuNPs, a magnetic resonance imaging (MRI) agent because of the presence of Fe3O4, and a temperature sensor because of the presence of UCNP, and in photothermal heating because of the presence of UCNP@SiO2@AuNPs and AC magnetic-based hyperthermia because of the presence of Fe3O4.