Glycogen synthase kinase 3β (GSK-3β) is a ubiquitous serine/threonine protein kinase involved in a number of signaling pathways. Previous studies have demonstrated a role for GSK-3β in the synaptic plasticity underlying dopamine-associated behaviors and diseases. Drug sensitization is produced by repeated exposure to the drug and is thought to reflect neuroadaptations that contribute to addiction. However, the role of GSK-3β in cocaine-induced behavior sensitization has not been examined. The present study investigated the effects of chronic cocaine exposure on GSK-3β activity in the nucleus accumbens (NAc) and determined whether changes in GSK-3β activity in the NAc are associated with cocaine-induced locomotor sensitization. We also explored whether blockade of GSK-3β activity in the NAc inhibits the initiation and expression of cocaine-induced locomotor sensitization in rats using systemic or brain region-specific administration of the GSK-3β inhibitors lithium chloride (LiCl) and SB216763. GSK-3β activity in the NAc core, but not NAc shell, increased after chronic cocaine (10 mg/kg, i.p.) administration. The initiation and expression of cocaine-induced locomotor sensitization was attenuated by systemic administration of LiCl (100 mg/kg, i.p.) or direct infusion of SB216763 (1 ng/side) into the NAc core, but not NAc shell. Collectively, these results indicate that GSK-3β activity in the NAc core, but not NAc shell, mediates the initiation and expression of cocaine-induced locomotor sensitization, suggesting that GSK-3β may be a potential target for the treatment of cocaine addiction.