Lanthanide-activated upconversion nanoparticles (UCNPs) have been widely applied for bioimaging and sensing. Their biomedical applications are usually limited by the thermal effect accompanied by UC emission, as most efficient UCNPs have to be excited at 980 nm, where strong absorption of water is located. We reported a core–shell NaYF₄:Yb³⁺, Tm³⁺@NaYF₄:Yb³⁺, Nd³⁺ nanosystem for temperature sensing with a minimized heating effect. Under excitation by an 808 nm laser, energy transfer from Nd³⁺ in the shell to Yb³⁺ in the shell then to Yb³⁺ in the core leads to the strong visible UC emission of Tm³⁺ at wavelengths of 450, 475, 645 and 696 nm. Based on a ratiometric approach, the nanosystem demonstrates a high sensitivity of up to 1.55% K⁻¹ at a temperature range between 313 and 553 K. As both excitation and emission light can be modulated in the biological window, the developed nanosystem may have potential applications in remote optical temperature sensing in biomedical fields.