Appropriate Wnt morphogen secretion is required to control animal development and homeostasis. Although correct Wnt globular structure is essential for secretion, proteins that directly mediate Wnt folding and maturation remain uncharacterized. Here, we report that protein disulfide isomerase-1 (PDI-1), a protein-folding catalyst and chaperone, controls secretion of the Caenorhabditis elegans Wnt ortholog EGL-20. We find that PDI-1 function is required to correctly form an anteroposterior EGL-20/Wnt gradient during embryonic development. Furthermore, PDI-1 performs this role in EGL-20/Wnt-producing epidermal cells to cell-non-autonomously control EGL-20/Wnt-dependent neuronal migration. Using pharmacological inhibition, we further show that PDI function is required in human cells for Wnt3a secretion, revealing a conserved role for disulfide isomerases. Together, these results demonstrate a critical role for PDIs within Wnt-producing cells to control long-range developmental events that are dependent on Wnt secretion.