The formation of PtSe2‐layered films is reported in a large area by the direct plasma‐assisted selenization of Pt films at a low temperature, where temperatures, as low as 100 °C at the applied plasma power of 400 W can be achieved. As the thickness of the Pt film exceeds 5 nm, the PtSe2‐layered film (five monolayers) exhibits a metallic behavior. A clear p‐type semiconducting behavior of the PtSe2‐layered film (≈trilayers) is observed with the average field effective mobility of 0.7 cm2 V−1 s−1 from back‐gated transistor measurements as the thickness of the Pt film reaches below 2.5 nm. A full PtSe2 field effect transistor is demonstrated where the thinner PtSe2, exhibiting a semiconducting behavior, is used as the channel material, and the thicker PtSe2, exhibiting a metallic behavior, is used as an electrode, yielding an ohmic contact. Furthermore, photodetectors using a few PtSe2‐layered films as an adsorption layer synthesized at the low temperature on a flexible substrate exhibit a wide range of absorption and photoresponse with the highest photocurrent of 9 µA under the laser wavelength of 408 nm. In addition, the device can maintain a high photoresponse under a large bending stress and 1000 bending cycles. Phase‐engineered PtSe2‐layered films by a plasma‐assisted selenization process to highly sensitive, flexible, and wide‐spectrum photoresponse photodetectors are demonstrated. A full PtSe2 field effect transistor is also demonstrated where the thinner PtSe2, exhibiting a semiconducting behavior, is used as the channel material, and the thicker PtSe2, exhibiting a metallic behavior, is used as an electrode.