High-mobility low-hysteresis electrolyte-gated thin-film transistors were successfully fabricated using a diketopyrrolopyrrole (DPP) and benzotriazole (BTz) copolymer semiconductor and high-capacitance ion gels based on 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([EMI][TFSI]) and tetraoctyl phosphonium ([P8888]) [TFSI]. The DPP-BTz transistors gated with both ion gels operated effectively at low voltages below 1V with high on-to-off current ratios exceeding 105 and very low device hysteresis. Specifically, when the [EMI][TFSI] ion gel was employed as a gate dielectric layer, the DPP-BTz transistors exhibited a very high carrier-mobility value of 8.50 ± 1.09 cm2/Vs. In addition, device hysteresis of the DPP-BTz transistor was almost invariant to the voltage sweeping rate and was much lower than that of the poly(3-hexylthiophene) (P3HT) transistor under the same conditions. Overall, these results indicate that the performance of electrolyte-gated transistors can be improved using the DPP-BTz semiconductor.