Transient electronics are of great interest in medical implants and environmentally friendly devices. These electronic devices can be partially or fully dissolved in water in a controlled period of time. Herein, we show the fabrication and characterization of silver nanoparticle-based strain sensors on the water-soluble nanofibril paper, a cellulose nanocrystal (CNC) produced flexible film, by stencil lithography. This is the first demonstration of using polydimethylsiloxane (PDMS) stencils to pattern metal electrodes on water-soluble substrates. For the large London dispersion force (LDF) between the PDMS stencil and the flexible substrate, the stencil can conformably cover the substrate and prevent metal diffusion into the area under the stencil when metal electrodes are patterned on water-soluble nanofibril papers. PDMS stencil lithography avoids damage to the cellulose nanofibril paper, which conventional photolithography processes lead to, without compromising the resolution in patterning. Our strain sensors have a high sensitivity with a gauge-factor of over 50 in strain testing, which is the highest among reported strain sensors fabricated on water-soluble substrates.