Thallium (Tl) is an extremely toxic trace element with high bioaccumulation. Selective and ultrafast Tl⁺ removal from water is critical to the safety of water resources but remains a grand challenge. Here, by intercalating KOH into multilayered Ti₃C₂T_x MXene, an efficient adsorbent with coordination/cation exchangeable sites (KOH@Ti₃C₂T_x) has been developed for Tl⁺ removal. Almost 99.6% of Tl⁺ could be removed over optimal KOH@Ti₃C₂T_x in just 10 s, and the maximal uptake capacity was up to 238.1 mg g⁻¹, which is 3.8-fold larger than that of the pristine Ti₃C₂T_x (61.4 mg g⁻¹). Moreover, benefitting from a wider working pH window and excellent selectivity, KOH@Ti₃C₂T_x could quickly remove almost all Tl⁺ from actual wastewater, demonstrating great potential for water purification. Control experiments and calculations jointly demonstrated that the boosted performance can be attributed to the synergistic effect of the abundant hydroxyl groups and exchangeable K⁺ sites, in which the hydroxyl groups can capture Tl⁺via coordination while the K⁺/Tl⁺ cation exchange contributes to selectivity and fast uptake kinetics. This work highlights the interaction mechanism of active sites and metal ions, providing fundamental guidance to the selective and rapid removal of heavy metals from water, especially in emergency disposal of heavy metal pollution accidents.