In this study, we demonstrate a method to fabricate a guided-mode resonance (GMR) device on a flexible and transparent low-density polyethylene (LDPE) film and present the measurement results of this device as a pressure sensor. A simple thermal-nanoimprinting process was used to fabricate a grating structure on the LDPE film substrate. This very flexible film was attached to a glass plate using an adhesive and sacrificial layer for coating high-refractive-index titanic oxide on the grating surface to form the GMR device. The LDPE-GMR device was equipped with a gas chamber to act as a pressure sensor. When the pressure inside the chamber was increased, the grating period of the GMR sensor also increased, resulting in a shift in the resonance angle of the GMR device. Owing to the higher flexibility of the LDPE film, a better pressure detection sensitivity and resolution can be obtained. Using the transmitted-intensity detection approach, we show that the transmitted laser power changes proportionally with the pressure increase. The experimental results showed that the LDPE-GMR pressure sensor could achieve a sensitivity of 8.27 µW/mbar and a limit of detection of 0.012 mbar at a power meter noise of 0.1 µW.