Emerging flaviviral pathogens represent a significant current and projected global health crisis. Dengue virus (DENV) is the leading disease-causing arbovirus in the world, with over 390 million infections annually, resulting in more than 500,000 severe cases requiring hospitalization. Adding to this burden, the recent global spread of the related Zika virus (ZIKV) has caught governments and aid agencies by surprise and resulted in the WHO declaration of a global health emergency. The co-circulation of these two related viruses poses significant complications for accurate diagnosis as well as vaccine design. However, the similarities between the two viruses also offers the potential for the development of future treatments and vaccines that could offer broad protection against these important human pathogens. We investigated the potential for cross-reactive antibodies that bind the nonstructural protein 1 (NS1) from both DENV and ZIKV with the aim of investigating selected antibodies through both structural and functional approaches. Here we present the identification of a highly cross-reactive Mab capable of binding both DENV and ZIKV NS1 protein which is also able to block NS1 induced leak both in vitro and in vivo. Peptide mapping and proteolytic product analysis revealed that this antibody recognizes a region within the c-terminal domain of NS1 (NS1c). Using cryoelectron microscopy (Cryo-EM) single particle analysis of the antibody together with both the soluble secreted hexameric form of NS1 as well as the dimeric NS1c subdomain we have revealed the epitope and stoichiometric basis for the recognition of NS1. These findings reveal a key cross-reactive epitope within NS1 that has significant implications for diagnostic development and also provide the foundation for new antivirals and vaccine design to counter both DENV and ZIKV.