Among the heavy metal ions, copper(II) can cause eye and liver damage at high uptake. The existence of copper ions (Cu²⁺) even with an ultralow concentration of less than 0.1 μg g⁻¹ can be toxic to living organisms. Thus, it is highly desirable to develop efficient adsorbents to remove Cu²⁺ from aqueous solutions. In this work, without any surface functionalization or pretreatment, a water-stable zeolitic imidazolate framework (ZIF-8) synthesized at room temperature is directly used as a highly efficient adsorbent for removal of copper ions from aqueous solutions. To experimentally unveil the adsorption mechanism of Cu²⁺ by using ZIF-8, we explore various effects from a series of important factors, such as pH value, contact time, temperature and initial Cu²⁺ concentration. As a result, ZIF-8 nanocrystals demonstrate an unexpected high adsorption capacity of Cu²⁺ and high removal efficiency for both high and low concentrations of Cu²⁺ from water. Moreover, ZIF-8 nanocrystals possess fast kinetics for removing Cu²⁺ with the adsorption time of less than 30 min. In addition, the pH of the solution ranging from 3 to 6 shows little effect on the adsorption of Cu²⁺ by ZIF-8. The adsorption mechanism is proposed for the first time and systematically verified by various characterization techniques, such as TEM, FTIR, XPS, XRD and SEM.