Direct phosphorylation of GluA1 by PKC controls α‐amino‐3‐hydroxy‐5‐methyl‐isoxazole‐4‐propionic acid (AMPA) receptor (AMPAR) incorporation into active synapses during long‐term potentiation (LTP). Numerous signalling molecules that involved in AMPAR incorporation have been identified, but the specific PKC isoform(s) participating in GluA1 phosphorylation and the molecule triggering PKC activation remain largely unknown. Here, we report that the atypical isoform of PKC, PKCλ, is a critical molecule that acts downstream of phosphatidylinositol 3‐kinase (PI3K) and is essential for LTP expression. PKCλ activation is required for both GluA1 phosphorylation and increased surface expression of AMPARs during LTP. Moreover, p62 interacts with both PKCλ and GluA1 during LTP and may serve as a scaffolding protein to place PKCλ in close proximity to facilitate GluA1 phosphorylation by PKCλ. Thus, we conclude that PKCλ is the critical signalling molecule responsible for GluA1‐containing AMPAR phosphorylation and synaptic incorporation at activated synapses during LTP expression.