We, for the first time, report potassium storage in carbon-modified Li4Ti5O12 spinel (C-LTO), where the carbon coating increases the electrical conductivity from ~10−7 to ~10−1 S cm−1. C-LTO provides a high initial charge capacity of approximately 221 mAh g −1 at 0.2C (34 mA g −1) with about 77% retention for 200 cycles. Pre-potassiation of C-LTO electrode successfully improved initial Coulombic efficiency. The excellent electrode performance is further emphasized at high rate (3.2C, 544 mA g −1), with an initial capacity of 130 mAh g −1 and 70% retention for 1000 cycles. During the reaction in K cells, the incorporation of K+ ions into the cubic spinel Li4Ti5O12 induces a biphasic reaction, namely cubic rock salt K-rich K6LiTi5O12 and Li-rich Li7Ti5O12 phases accompanied by Ti4+/3+ redox, as confirmed by in-situ X-ray diffraction and ex-situ X-ray absorption analyses. The original Li4Ti5O12 spinel is recovered upon depotassiation. In addition, full cells paired with a P3-K0.5[Mn0.8Fe0.1Ni0.1]O2 cathode demonstrate the feasibility of applying the C-LTO electrode as an anode for high-rate and long-term potassium storage.