Frameshifting of mRNA during translation provides a strategy to expand the coding repertoire of cells and viruses. Where and how in the elongation cycle +1-frameshifting occurs remains poorly understood. We captured six ∼3.5-Å-resolution cryo-EM structures of ribosomal elongation complexes formed with the GTPase elongation factor G (EF-G). Three structures with a +1-frameshifting-prone mRNA reveal that frameshifting takes place during translocation of tRNA and mRNA. Prior to EF-G binding, the pre-translocation complex features an in-frame tRNA-mRNA pairing in the A site. In the partially translocated structure with EF-G, the tRNA shifts to the +1-frame codon near the P site, whereas the freed mRNA base bulges between the P and E sites and stacks on the 16S rRNA nucleotide G926. The ribosome remains frameshifted in the nearly post-translocation state. Our findings demonstrate that the ribosome and EF-G cooperate to induce +1 frameshifting during mRNA translocation.