Despite the central role PSD-95 plays in anchoring postsynaptic AMPARs, how PSD-95 itself is tethered to postsynaptic sites is not well understood. Here we show that the F-actin binding protein alpha-actinin binds to the very N terminus of PSD-95. Knockdown (KD) of alpha-actinin phenocopies KD of PSD-95. Mutating lysine at position 10 or lysine at position 11 of PSD-95 to glutamate, or glutamate at position 53 or glutamate and aspartate at positions 213 and 217 of alpha-actinin, respectively, to lysine impairs, in parallel, PSD-95 binding to alpha-actinin and postsynaptic localization of PSD-95 and AMPARs. These experiments identify alpha-actinin as a critical PSD-95 anchor tethering the AMPAR-PSD-95 complex to postsynaptic sites.