Two-dimensional (2D) Ruddlesden–Popper (RP) perovskites are emerging materials for light-emitting applications. Unfortunately, their desirable narrowband emission coexists with broadband emissions, which limits the color quality and performance of the light source. However, the origin of such broadband emission in ⟨100⟩-oriented perovskites is still under debate. Here, we experimentally and theoretically demonstrate that unlike ⟨110⟩-oriented RP perovskites, the broadband emission of the 2D ⟨100⟩-oriented RP (PEA)2PbI4 (PEA = C6H5C2H4NH3+) perovskites originates from defect-related luminescence centers. We find that the broadband emission of this prototype 2D structure can be largely suppressed by using excess PEAI treatment. Density functional theory (DFT) calculations indicate that iodine (I) vacancies both in the bulk and on the surface are responsible for the broadband emission. We attribute the decreased broadband emission after PEAI treatment to the passivation of both undercoordinated Pb2+ ions on the surface and I vacancies in the bulk through I– ion migration.