Degradation of folding- or assembly-defective proteins by the endoplasmic reticulum–associated degradation (ERAD) ubiquitin ligase, Hrd1, is facilitated by a process that involves recognition of demannosylated N-glycans by the lectin OS-9/XTP3-B via the adaptor protein SEL1L. Most of our knowledge of the machinery that commits proteins to this fate in metazoans comes from studies of overexpressed mutant proteins in heterologous cells. In this study, we used mass spectrometry to identify core-glycoslyated CD147 (CD147(CG)) as an endogenous substrate of the ERAD system that accumulates in a complex with OS-9 following SEL1L depletion. CD147 is an obligatory assembly factor for monocarboxylate transporters. The majority of newly synthesized endogenous CD147(CG) was degraded by the proteasome in a Hrd1-dependent manner. CD147(CG) turnover was blocked by kifunensine, and interaction of OS-9 and XTP3-B with CD147(CG) was inhibited by mutations to conserved residues in their lectin domains. These data establish unassembled CD147(CG) as an endogenous, constitutive ERAD substrate of the OS-9/SEL1L/Hrd1 pathway.