Anion-π interaction has been found to play a key role in interfacial phenomena. In this study, we evaluated the anion-π interactions at the solid/water interface. Anion adsorption originating from anion-π interaction at the interfaces followed the hydration energy and the presence of conjugated systems of the anions by the QCM measurements. Force curve measurements revealed that the single-molecule force of anion-π interaction between an NDI unit and the negatively charged surface of the cantilever was ∼40 pN. To the best of our knowledge, this is the first example of obtaining a single-molecule force for anion-π interactions.