Relative humidity (RH) plays an important role in almost every industrial field. Thus, the detection of RH is of great significance in these fields. Terahertz (THz) waves are extremely sensitive to the changes in RH because water absorbs strongly in this electromagnetic band. In this paper, a RH sensor based on THz metasurfaces combined with polyvinyl alcohol (PVA) is proposed. Different from the conventional metasurface sensor, our sensor includes a PVA layer that is sandwiched between the metal structure and the substrate. The improved design is able to enhance the interaction between the electric field and the water molecules absorbed in the PVA layer, and the sensitivity of the humidity sensor can reach 0.34 GHz/%RH, which is more than twice that of the conventional metasurface sensor. These results show that the proposed sensor can be used for the detection of RH with high reliability and high sensitivity, which open a new, to the best of our knowledge, avenue for RH sensing using THz spectroscopy.