SLC26A9 is a member of the SLC26 family of anion transporters, which is expressed at high levels in airway and gastric surface epithelial cells. The transport properties and regulation of SLC26A9, and thus its physiological function, are not known. Here we report that SLC26A9 is a highly selective Cl(-) channel with minimal OH(-)/HCO(3)(-) permeability that is regulated by the WNK kinases. Expression in Xenopus oocytes and simultaneous measurement of membrane potential or current, intracellular pH (pH(i)) and intracellular Cl(-) (Cl(-)(i)) revealed that expression of SLC26A9 resulted in a large Cl(-) current. SLC26A9 displays a selectivity sequence of I(-) > Br(-) > NO(3)(-) > Cl(-) > Glu(-), but it conducts Br(-) > Cl(-) > I(-) > NO(3)(-) > Glu(-), with NO(3)(-) and I(-) inhibiting the Cl(-) conductance. Similarly, expression of SLC26A9 in HEK cells resulted in a large Cl(-) current. Although detectable, OH(-) and HCO(3)(-) fluxes in oocytes expressing SLC26A9 were very small. Moreover, HCO(3)(-) had no discernable effect on the Cl(-) current, the reversal potential in the presence or absence of Cl(-)(o) and, importantly, HCO(3)(-) had no effect on Cl(-) fluxes. These findings indicate that SLC26A9 is a Cl(-) channel with minimal OH(-)/HCO(3)(-) permeability. Co-expression of SLC26A9 with the WNK kinases WNK1, WNK3 or WNK4 inhibited SLC26A9 activity, and the inhibition was independent of WNK kinase activity. Immunolocalization in oocytes and cell surface biotinylation in HEK cells indicated that the WNK-mediated inhibition of SLC26A9 activity is caused by reduced SLC26A9 surface expression. Expression of SLC26A9 in the airway and the response of the WNKs to homeostatic stress raise the possibility that SLC26A9 serves to mediate the response of the airway to stress.