Messenger RNA (mRNA) homeostasis represents an essential part of gene expression, in which the generation of mRNA by RNA polymerase is counter-balanced by its degradation by nucleases. The conserved 5'-to-3' exoribonuclease Xrn1 has a crucial role in eukaryotic mRNA homeostasis by degrading decapped or cleaved mRNAs post-translationally and, more surprisingly, also co-translationally. Here we report that active Xrn1 can directly and specifically interact with the translation machinery. A cryo-electron microscopy structure of a programmed Saccharomyces cerevisiae 80S ribosome-Xrn1 nuclease complex reveals how the conserved core of Xrn1 enables binding at the mRNA exit site of the ribosome. This interface provides a conduit for channelling of the mRNA from the ribosomal decoding site directly into the active center of the nuclease, thus separating mRNA decoding from degradation by only 17 ± 1 nucleotides. These findings explain how rapid 5'-to-3' mRNA degradation is coupled efficiently to its final round of mRNA translation.