Because they possess excellent visible light absorption properties, lead-free colloidal copper-based chalcogenide quantum dots (QDs) have emerged in photoelectronic fields. By means of localized surface plasmonic resonance (LSPR), the absorption properties of QDs can be enhanced. In this paper, we fabricate a lead-free ${\mathrm{CuInSe}}_2$ QD field effect phototransistor (FEpT) by utilizing the LSPR enhancement of Au nanoparticles (NPs). The plasmonic FEpT demonstrates responsivity up to $2.7\text{μA}·{\text{W}}^1$ and a specific detectivity of $7\times {10}^3$ Jones at zero bias under illumination by a 532 nm laser, values that are enhanced by approximately 200% more than devices without Au NPs. Particularly, the FEpT exhibits a multi-wavelength response, which is photoresponsive to 405, 532, and 808 nm irradiations, and presents stability and reproducibility in the progress of ON–OFF cycles. Furthermore, the enhancement induced by Au NP LSPR can be interpreted by finite-difference time domain simulations. The low-cost solution-based process and excellent device performance strongly underscore lead-free ${\mathrm{CuInSe}}_2$ QDs as a promising material for self-powered photoelectronic applications, which can be further enhanced by Au NP LSPR.