<p>Propagating surface plasmon resonance (PSPR) is an optical phenomenon that occurs at the metal/dielectric interface and has been widely used in organic solar cells. However, its application in organic phototransistors (OPTs) is still rare. Therefore, in this study, we investigated the use of PSPR for enhancing photoresponses of a pentacene thin-film OPT. A pentacene film-based device consisting of a grating substrate/Al/Al₂O₃/fluoropolymer (CYTOP)/pentacene/Au-gap electrode/CYTOP was fabricated and examined. PSPR excitation in the device was confirmed via light transmission spectroscopy, dispersion curves, and finite-difference time-domain simulations. The fabricated device works as a field-effect transistor (FET) with a low operational voltage of − 1.36&#xa0;V. Compared to s-polarized illumination with an intensity of 1.23 mW/cm², the photoresponsivity and specific detectivity of the OPT increased from 82 to 105 μA/W and from 2.02 × 10⁷ Jones to 2.57 × 10⁷ Jones under p-polarized light inducing PSPR, respectively. This corresponds to an enhancement of approximately 30%, which is attributed to the excitation of PSPR along the Al film. This study demonstrates that utilizing PSPR excitations along a grating metal surface is a promising approach for enhancing the photodetection performance of organic thin-film phototransistors.</p>