A novel relative humidity (RH) sensor, based on an in-line microfiber interferometer (MFI), is proposed and demonstrated in an experiment. The microfiber interferometer is fabricated by tapering multi-core fiber down to the micrometer scale in diameter, and the coupling occurs between the center core mode and the multiple side core modes. To enhance the sensing performance, the microfiber sensor is fixed as U-shaped. The variation of RH is monitored by observing the wavelength shift of the transmission spectrum. The experimental results show that the sensor has a good linear response to RH. A thinner MFI diameter of 8.52 µm can offer a better RH sensitivity of 59.8 pm/(%RH) in the range of 35-95% RH. When temperature changes from 35°C to 85°C, the temperature sensitivity of the sensor is 4.2 pm/°C, which demonstrates that the sensor is insensitive to temperature. The simultaneous measurement of temperature and RH can be realized by cascading with fiber Bragg grating. The microfiber sensor is easy to fabricate and does not have any functional coating, which makes it widely used in accurate RH measuring.