A novel, to the best of our knowledge, in-fiber temperature sensor based on green up-conversion (UC) luminescence in an Er3+-Yb3+ co-doped tellurite glass microsphere is described. The tellurite glass microsphere is located firmly inside a suspended tri-core hollow-fiber (STCHF) structure. The pump light launched via a single-mode fiber (SMF) is passed through a section of multimode fiber, which is fusion spliced between the SMF and the STCHF into the cores suspended inside the hollow fiber and coupled into the microsphere. Green and red UC emissions of the Er3+ ions are observed using 980 nm pump excitation. The temperature-sensing capability of the tellurite glass microsphere is based on the thermally coupled effect between the upper energy levels responsible for green emissions at 528 nm and 549 nm. The resulting fluorescence intensity ratio, depending on the surrounding temperature range from 303 K to 383 K, is experimentally determined, and a maximum sensitivity of 5.47×10-3 K-1 is demonstrated. This novel in-fiber microsphere-resonator-based device is highly integrated and has the additional advantages of ease of fabrication, compact structure, and low fabrication cost and therefore has great application potential in integrated optical sources including lasers.