We report optical gating at the pentacene/lead sulfide (PbS) functional interface through which mobile carriers are confined in the pentacene layer close to the PbS colloidal quantum dot (CQD) layer. Using a bottom-contact pentacene/PbS field effect transistor (FET) structure, hole doping in a pentacene layer is demonstrated and the mechanism by which mobile carriers are created is elucidated by probing threshold voltage shift in the FET and the pentacene/PbS interfacial trap density. A large threshold voltage shift under selective illumination (780 nm) of the PbS CQD layer is interpreted as signature of hole transfer from the PbS to the pentacene. Electron trapping at the pentacene/PbS interface is suggested to be involved in the optical gating process.