Updated publication reference for PubMed record(s): 31640799. Human cells have evolved multiple quality control mechanisms to ensure protein homeostasis by detecting aberrant mRNAs and protein products which are condemned for rapid decay. Translating ribosomes that run into poly(A) tails are terminally stalled, followed by ribosome recycling and rapid decay of the truncated nascent polypeptide via the ribosome-associated quality control (RQC). Here, we demonstrate that the conserved RNA-binding E3 ubiquitin ligase Makorin Ring Finger Protein 1 (MKRN1) acts as a novel sensor of poly(A) sequences to initiate ribosome stalling in RQC. Using individual-nucleotide resolution UV crosslinking and immunoprecipitation (iCLIP), we show that MKRN1 is positioned upstream of A-rich stretches and poly(A) tails in mRNAs via interaction with the cytoplasmic poly(A)-binding protein (PABP). Ubiquitin remnant profiling uncovers PABP and ribosomal protein RPS10 as well as multiple translational regulators as main targets of MRKN1-mediated ubiquitylation. The modified lysine in RPS10 is distinct from the main target sites of ZNF598, the second E3 ubiquitin ligase involved in initial ribosome stalling, indicating that multiple modifications need to converge to prevent promiscuous ubiquitylation of lysine-translating ribosomes. We propose that MKRN1 serves as a first line of poly(A) recognition at the RNA level to prevent erroneous protein products and maintain proteome integrity.