Diketopyrrolopyrrole (DPP) is one of the most promising building blocks for constructing polymer semiconductors with high charge-carrier mobilities in organic field-effect transistors (OFETs). In this study, a novel DPP-based conjugated polymer, PDPPy-BDD, was designed and synthesized. The ambipolar field-effect transistor characteristics were realized with the average hole and electron mobilities of 3.5 × 10–3 and 3.07 × 10–2 cm2 V−1 s−1, respectively. Both the hole and electron mobilities could be successfully enhanced by using a tetramethylammonium iodide (NMe4I) additive. Such an enhancement was attributed to the formation of stronger interchain π–π stackings, the weakening of the face-on packing orientation in the thin film state, and the higher channel conductivities in the OFETs.