FAM46C is a non-canonical poly(A) polymerase uniquely mutated in up to 20% of multiple myeloma (MM) patients, implying a tissue-specific tumor suppressor function. Here, we report that FAM46C selectively stabilizes mRNAs encoding endoplasmic reticulum (ER)-targeted proteins, thereby concertedly enhancing the expression of proteins that control ER protein import, folding, N-glycosylation, and trafficking and boosting protein secretion. This role requires the interaction with the ER membrane resident proteins FNDC3A and FNDC3B. In MM cells, FAM46C expression raises secretory capacity beyond sustainability, inducing ROS accumulation, ATP shortage, and cell death. FAM46C activity is regulated through rapid proteasomal degradation or the inhibitory interaction with the ZZ domain of the autophagic receptor p62 that hinders its association with FNDC3 proteins via sequestration in p62+ aggregates. Altogether, our data disclose a p62/FAM46C/FNDC3 circuit coordinating sustainable secretory activity and survival, providing an explanation for the MM-specific oncosuppressive role of FAM46C and uncovering potential therapeutic opportunities against cancer. Display omitted •In myeloma cells, FAM46C boosts ER growth and Ig secretion beyond sustainability•The ER-expanding activity of FAM46C requires its interaction with FNDC3 proteins•FAM46C abundance is tightly regulated by UPS-operated proteolysis•p62 compensates for UPS insufficiency by sequestering FAM46C away from the ER Fucci et al. show that the poly(A) polymerase FAM46C acts as a multiple myeloma-specific tumor suppressor, increasing secretory capacity and antibody production beyond sustainability via its interaction with endoplasmic reticulum transmembrane FNDC3 proteins. Moreover, its activity is restricted through proteasomal degradation or p62-dependent aggregation and sequestration from FNDC3 proteins.