We demonstrated a simple fabrication of polymeric diaphragm pressure sensors by embedding a commercial thin-film strain gauge in a polymeric diaphragm. Two different polymer materials, polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET), were employed for fabrication. Performance of pressure sensors fabricated with two different polymers including sensitivity, linearity, and long-term stability was thoroughly compared using a characterization setup operating in either compressed air or water. Pressure sensitivities were similar for air and water environments regardless of the polymer material. However, the sensitivity and resolution of PET pressure sensors were about 2.5 times better than those of PDMS pressure sensors. Moreover, significant degradation of PDMS pressure sensors was observed over time while PET pressure sensors exhibited good stability in water. Overall, PET is better suited for making liquid pressure sensors than PDMS.