Anoctamin 1 (ANO1)/transmembrane protein 16A (TMEM16A) is a calcium-activated anion channel that may play a role in HCO₃⁻ secretion in epithelial cells. Here, we report that the anion selectivity of ANO1 is dynamically regulated by the Ca²⁺/calmodulin complex. Whole-cell current measurements in HEK 293T cells indicated that ANO1 becomes highly permeable to HCO₃⁻ at high [Ca²⁺]_i. Interestingly, this result was not observed in excised patches, indicating the involvement of cytosolic factors in this process. Further studies revealed that the direct association between ANO1 and calmodulin at high [Ca²⁺]_i is responsible for changes in anion permeability. Calmodulin physically interacted with ANO1 in a [Ca²⁺]_i-dependent manner, and addition of recombinant calmodulin to the cytosolic side of excised patches reversibly increased P_HCO3/P_Cl. In addition, the high [Ca²⁺]_i-induced increase in HCO₃⁻ permeability was reproduced in mouse submandibular gland acinar cells, in which ANO1 plays a critical role in fluid secretion. These results indicate that the HCO₃⁻ permeability of ANO1 can be dynamically modulated and that ANO1 may play an important role in cellular HCO₃⁻ transport, especially in transepithelial HCO₃⁻ secretion.