Serotonin (5-hydroxytryptamine; 5-HT) signaling through the 5-HT_2Creceptor (5-HT_2CR) is essential in normal physiology, whereas aberrant 5-HT_2CR function is thought to contribute to the pathogenesis of multiple neural disorders. The 5-HT_2CR interacts with specific protein partners, but the impact of such interactions on 5-HT_2CR function is poorly understood. Here, we report convergent cellular and behavioral data that the interaction between the 5-HT_2CR and protein phosphatase and tensin homolog (PTEN) serves as a regulatory mechanism to control 5-HT_2CR-mediated biology but not that of the closely homologous 5-HT_2AR. A peptide derived from the third intracellular loop of the human 5-HT_2CR [3L4F (third loop, fourth fragment)] disrupted the association, allosterically augmented 5-HT_2CR-mediated signaling in live cells, and acted as a positive allosteric modulator in ratsin vivo. We identified the critical residues within an 8 aa fragment of the 3L4F peptide that maintained efficacy (within the picomolar range) in live cells similar to that of the 3L4F peptide. Last, molecular modeling identified key structural features and potential interaction sites of the active 3L4F peptides against PTEN. These compelling data demonstrate the specificity and importance of this protein assembly in cellular events and behaviors mediated by 5-HT_2CR signaling and provide a chemical guidepost to the future development of drug-like peptide or small-molecule inhibitors as neuroprobes to study 5-HT_2CR allostery and therapeutics for 5-HT_2CR-mediated disorders.