Updated project metadata. Impaired secretion of an essential blood coagulation factor fibrinogen leads to hepatic fibrinogen storage disease (HFSD), characterized by the presence of fibrinogen-positive inclusion bodies and hypofibrinogenemia. However, the molecular mechanisms underlying the biogenesis of fibrinogen in the endoplasmic reticulum (ER) remains unexplored. Here we uncovered a key role of SEL1L-HRD1 complex of ER-associated degradation (ERAD) in the formation of aberrant inclusion bodies, and the biogenesis of nascent fibrinogen protein complex in hepatocytes. Serendipitously, acute or chronic deficiency of SEL1L-HRD1 ERAD, but not UPR sensor IRE1α, in the hepatocytes led to the formation of hepatocellular inclusion bodies. Proteomics studies followed by biochemical assays revealed fibrinogen, not albumin or α1-antitrypsin, as a major component of the inclusion bodies. Degradation of misfolded endogenous fibrinogen Aα, Bβ, and γ chains by SEL1L-HRD1 ERAD was required for the formation of a functional fibrinogen complex in the ER. Providing clinical relevance of these findings, SEL1L-HRD1 ERAD indeed degraded and thereby attenuated the pathogenicity of a disease-causing fibrinogen γ mutant. Together, this study demonstrates an essential role of SEL1L-HRD1 ERAD in fibrinogen biogenesis and provides novel insights into the pathogenesis of protein-misfolding diseases.