The excitability of the central nervous system depends largely on the surface density of neurotransmitter receptors. The endocannabinoid receptor 1 (CB R) and the metabotropic glutamate receptor mGlu R are expressed pre-synaptically where they reduce glutamate release into the synaptic cleft. Recently, the CB R interacting protein cannabinoid receptor interacting protein 1a (CRIP1a) was identified and characterized to regulate CB R activity in neurons. However, underlying molecular mechanisms are largely unknown. Here, we identified a common mechanism used by CRIP1a to regulate the cell surface density of two different types of G-protein coupled receptors, CB R and mGlu R. Five amino acids within the CB R C-terminus were required and sufficient to reduce constitutive CB R endocytosis by about 72% in the presence of CRIP1a. Interestingly, a similar sequence is present in mGlu R and consistently, endocytosis of mGlu R depended on CRIP1a, as well. Docking analysis and molecular dynamics simulations identified a conserved serine in CB R (S468) and mGlu R (S894) that forms a hydrogen bond with the peptide backbone of CRIP1a at position R82. In contrast to mGlu R, the closely related mGlu R splice-variant carries a lysine (K894) at this position, and indeed, mGlu R endocytosis was not affected by CRIP1a. Chimeric constructs between CB R, mGlu R, and mGlu R underline the role of the identified five CRIP1a sensitive amino acids. In summary, we suggest that CRIP1a negatively regulates endocytosis of two different G-protein coupled receptor types, CB R and mGlu R.