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 PHCO3/PCl. 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.