Dynamic regulation of ion channel interactions with the cytoskeleton mediates aspects of synaptic plasticity, yet mechanisms for this process are largely unknown. Here, we report that two inwardly rectifying K⁺ channels, Kir 2.1 and 2.3, bind to PSD-95, a cytoskeletal protein of postsynaptic densities that clusters NMDA receptors and voltage-dependent K⁺ channels. Kir 2.3 colocalizes with PSD-95 in neuronal populations in forebrain, and a PSD-95/Kir 2.3 complex occurs in hippocampus. Within the C-terminal tail of Kir 2.3, a serine residue critical for interaction with PSD-95, is also a substrate for phosphorylation by protein kinase A (PKA). Stimulation of PKA in intact cells causes rapid dissociation of the channel from PSD-95. This work identifies a physiological mechanism for regulating ion channel interactions with the postsynaptic density.