This article presents the design and fabrication process of a highly miniaturized nanomechanical Si-polymer composite membrane-type internal-stress sensor (MIS) with piezoresistive elements for humidity detection. The 500-μm ² area dimension sensor consists of a thin Si-polymer composite membrane supported by two piezoresistive Si beams. The composite membrane is fabricated as follows: First, 1-μm-wide width Si slits with 1 μm separation gaps are formed by using photolithography and deep reactive-ion etching, and then, the Si slits are filled with a functional polymer (methyl methacrylate based acrylate resin: OLESTER ^™ Q155). This composite device acts as an absorber of vapor molecules and, consequently, generates stress. The fabricated sensor in a humidity-controlled chamber shows a relative resistance change ΔR/R _device = 0.6% for 58% relative humidity (RH) change and a highly linear steady response of 5.2 mV/%RH up to 70% RH change with sensing resolution of 0.5% humidity. The measured polymer expansion ratio of ε _p = 2.4 × 10 ^-3 is consistent with the theoretical estimation using the weight fraction of water adsorbed in the polymer.