Dengue disease is caused by four different flavivirus1 serotypes, which infect 390 million people yearly with 25% symptomatic cases2 and for which no licensed vaccine is available. Recent phase III vaccine trials showed partial protection, and in particular no protection for dengue virus serotype 2 (refs 3, 4). Structural studies so far have characterized only epitopes recognized by serotype-specific human antibodies5, 6. We recently isolated human antibodies potently neutralizing all four dengue virus serotypes7. Here we describe the X-ray structures of four of these broadly neutralizing antibodies in complex with the envelope glycoprotein E from dengue virus serotype 2, revealing that the recognition determinants are at a serotype-invariant site at the E-dimer interface, including the exposed main chain of the E fusion loop8 and the two conserved glycan chains. This ‘E-dimer-dependent epitope’ is also the binding site for the viral glycoprotein prM during virus maturation in the secretory pathway of the infected cell9, explaining its conservation across serotypes and highlighting an Achilles’ heel of the virus with respect to antibody neutralization. These findings will be instrumental for devising novel immunogens to protect simultaneously against all four serotypes of dengue virus.