An unprecedented organocatalytic cascade aldimine condensation/[1,6]-hydride transfer/Mannich-type cyclization of indole-2-carbaldehydes with o-aminoanilines was developed to assemble polycyclic indole-2,3-fused diazocanes in one step. This novel methodology features high step- and atom-economy, is redox-neutral and metal-free, and occurs at room temperature. It opens a new window for the application of hydride transfer chemistry in constructing medium-sized ring structures.