We present a flexible humidity sensor comprising a reduced graphene oxide/polyaniline (RGO/PANI) composite film on a polypropylene filter paper substrate. Graphene oxide was reduced with sodium borohydride (NaBH4), then PANI was prepared by the oxidative polymerization of aniline to produce the RGO/PANI composite. The suspension of the as-prepared RGO/PANI composite was simply filtered on the paper substrate for sensor fabrication. The structure and morphology of the obtained composite were investigated by SEM, Fourier transform IR spectroscopy, and Raman spectroscopy. The experimental results revealed that the RGO/PANI composite film on the paper substrate can be used for humidity sensing with low hysteresis, good repeatability, and a large response in a broad humidity range. Moreover, our sensor has the potential to be applied in wearable devices because of the flexible substrate.