The Iodic acid‑iodous acid (HIO₃–HIO₂) binary system is a highly efficient nucleation pathway in marine environments, yet it cannot fully explain observed new particle formation (NPF) events. Combining quantum chemical calculations and kinetic simulations, we demonstrate that ethylenediamine (EDA)—a strong diamine from anthropogenic and marine sources could synergistically enhance HIO₃-HIO₂ nucleation via a ternary mechanism. The result demonstrates that both EDA and HIO₂ accept protons from HIO₃ to stabilize initial clusters, and replacing HIO₂ with EDA molecules further reinforces cluster stability. At 278.15 K, EDA concentrations ([EDA]) exceeding 0.1 ppt, both typical of the marine atmosphere, the aerosol formation rate (J) of the HIO₃-HIO₂ system increases by over one order of magnitude under low iodine oxoacid concentrations. Comparing field observations with simulations using conditions from coastal/polar sites and estimated [EDA] elsewhere, we find that the HIO₃-HIO₂-EDA ternary mechanism significantly contributes to NPF in cold polar and clean coastal regions. Its simulated J values match field measurements better than those of the binary HIO₃-HIO₂ and HIO₃‑EDA systems, effectively reproducing observed NPF characteristics. This ternary system is expected to be a key contributor to marine NPF in regions with intensive marine industries, carbon capture activities, and coastal agricultural/industrial emissions. These findings broaden our understanding of multicomponent marine aerosol nucleation and highlight the need for future research on strongly basic precursors similar to EDA in promoting HIO₃-HIO₂ nucleation in complex marine environments.