A stimulation inducing long-term potentiation (LTP) of synaptic transmission induces a persistent expansion of dendritic spines, a phenomenon known as structural LTP (sLTP). We previously proposed that the formation of a reciprocally activating kinase-effector complex (RAKEC) between CaMKII and Tiam1, an activator of the small G-protein Rac1, locks CaMKII into an active conformation, which in turn maintains the phosphorylation status of Tiam1. This makes Rac1 persistently active, specifically in the stimulated spine. To understand the significance of the CaMKII-Tiam1 RAKEC in vivo, we generated a Tiam1 mutant knock-in mouse line in which critical residues for CaMKII binding were mutated into alanines. We confirmed the central role of this interaction on sLTP by observing that KI mice showed reduced Rac1 activity, had smaller spines and a diminished sLTP as compared to their wild-type littermates. Moreover, behavioral tests showed that the novel object recognition memory of these animals was impaired. We thus propose that the CaMKII-Tiam1 interaction regulates spine morphology in vivo and is required for memory storage.