We propose and investigate a highly sensitive fiber optic temperature sensor based on a two-mode-single-mode microfiber Sagnac loop (TS-MSL). The TS-MSL is fabricated by fused biconical tapering after a section of two-mode optical fiber is spliced between two sections of single-mode optical fibers, and it is packaged by the polydimethylsiloxane (PDMS) material. The experimental results reveal that the temperature sensing performance of the proposed sensor is effectively improved due to the optical field interference enhancement of TS-MSL as well as the photothermal sensitivity of the PDMS material. The temperature sensitivity of the proposed fiber sensor is up to 8.13 nm/℃, which is about 120 times higher than that of the ordinary single-mode microfiber Sagnac loop. With high sensitivity, the proposed temperature sensor is of high practical significance for accurate temperature monitoring in special occasions such as toxic gases and corrosive environments.