Proper function of the retina depends heavily on a specialized form of retinal glia called Müller cells. These cells carry out important homeostatic functions that are contingent on their polarized nature. Specifically, the Müller cell endfeet that contact retinal microvessels and the corpus vitreum show a tenfold higher concentration of the inwardly rectifying potassium channel K 4.1 than other Müller cell plasma membrane domains. This highly selective enrichment of K 4.1 allows K+ to be siphoned through endfoot membranes in a special form of spatial buffering. Here, we show that K 4.1 is enriched in endfoot membranes through an interaction with β1-syntrophin. Targeted disruption of this syntrophin caused a loss of K 4.1 from Müller cell endfeet without affecting the total level of K 4.1 expression in the retina. Targeted disruption of α1-syntrophin had no effect on K 4.1 localization. Our findings show that the K 4.1 aggregation that forms the basis for K+ siphoning depends on a specific syntrophin isoform that colocalizes with K 4.1 in Müller endfoot membranes.