Abstract Sialic acids are typically added to the end of glycoconjugates by sialyltransferases. Among the six ST8 α- N -acetyl-neuraminide α-2,8-sialyltransferases (ST8SIA) existing in adult brains, ST8SIA2 is a schizophrenia-associated gene. However, the in vivo substrates and physiological functions of most sialyltransferases are currently unknown. The ST8SIA3 is enriched in the striatum. Here, we showed that ablation of St8sia3 in mice ( St8sia3 -KO) led to fewer disialylated and trisialylated terminal glycotopes in the striatum of St8sia3 -KO mice. Moreover, the apparent sizes of several striatum-enriched G-protein-coupled receptors (GPCRs) (including the adenosine A 2A receptor (A 2A R) and dopamine D 1 /D 2 receptors (D 1 R and D 2 R)) were smaller in St8sia3 -KO mice than in WT mice. A sialidase treatment removed the differences in the sizes of these molecules between St8sia3 -KO and WT mice, confirming the involvement of sialylation. Expression of ST8SIA3 in the striatum of St8sia3 -KO mice using adeno-associated viruses normalized the sizes of these proteins, demonstrating a direct role of ST8SIA3. The lack of ST8SIA3-mediated sialylation altered the distribution of these proteins in lipid rafts and the interaction between D 2 R and A 2A R. Locomotor activity assays revealed altered pharmacological responses of St8sia3 -KO mice to drugs targeting these receptors and verified that a greater population of D 2 R formed heteromers with A 2A R in the striatum of St8sia3 -KO mice. Since the A 2A R-D 2 R heteromer is an important drug target for several basal ganglia diseases (such as schizophrenia and Parkinson’s disease), the present study not only reveals a crucial role for ST8SIA3 in striatal functions but also provides a new drug target for basal ganglia-related diseases.