AbstractHybrid‐perovskite‐based optoelectronic devices are demonstrating unprecedented growth in performance, and defect passivation approaches are highly promising routes to further improve properties. Here, the effect of the molecular ion BF₄⁻, introduced via methylammonium tetrafluoroborate (MABF₄) in a surface treatment for MAPbI₃ perovskite, is reported. Optical spectroscopy characterization shows that the introduction of tetrafluoroborate leads to reduced non‐radiative charge‐carrier recombination with a reduction in first‐order recombination rate from 6.5 × 10⁶ to 2.5 × 10⁵ s⁻¹ in BF₄⁻‐treated samples, and a consequent increase in photoluminescence quantum yield by an order of magnitude (from 0.5 to 10.4%). ¹⁹F, ¹¹B, and ¹⁴N solid‐state NMR is used to elucidate the atomic‐level mechanism of the BF₄⁻ additive‐induced improvements, revealing that the BF₄⁻ acts as a scavenger of excess MAI by forming MAI–MABF₄ cocrystals. This shifts the equilibrium of iodide concentration in the perovskite phase, thereby reducing the concentration of interstitial iodide defects that act as deep traps and non‐radiative recombination centers. These collective results allow us to elucidate the microscopic mechanism of action of BF₄⁻.