Translation affects messenger RNA stability and, in yeast, this is mediated by the Ccr4–Not deadenylation complex. The details of this process in mammals remain unclear. Here, we use cryogenic electron microscopy (cryo-EM) and crosslinking mass spectrometry to show that mammalian CCR4–NOT specifically recognizes ribosomes that are stalled during translation elongation in an in vitro reconstituted system with rabbit and human components. Similar to yeast, mammalian CCR4–NOT inserts a helical bundle of its CNOT3 subunit into the empty E site of the ribosome. Our cryo-EM structure shows that CNOT3 also locks the L1 stalk in an open conformation to inhibit further translation. CCR4–NOT is required for stable association of the nonconstitutive subunit CNOT4, which ubiquitinates the ribosome, likely to signal stalled translation elongation. Overall, our work shows that human CCR4–NOT not only detects but also enforces ribosomal stalling to couple translation and mRNA decay.