Defect management in perovskite solar cells (PSCs) via surface passivation has become a cornerstone in maximizing both stability and solar‐to‐electrical power conversion efficiency (PCE) of devices by reducing defect densities and/or improving energetic alignment between the perovskite and charge‐transporting layers. Despite this, few reports explore the use of roll‐to‐roll compatible technologies to deposit such interfacial treatments, limiting the applicability of passivation in real‐world contexts. In this work, iso‐butylammonium bromide (i‐BABr) is spray‐coated onto a Cs_0.15FA_0.85PbI_2.85Cl_0.15 perovskite surface which is deposited using gas‐assisted ultrasonic spray coating. It is found that i‐BABr treatments result in the formation of a quasi‐2D perovskite layer. The spray‐coated surface treatment results in an impressive 80 mV improvement in the median open‐circuit voltage with respect to untreated devices. Importantly, the spray‐coated passivation results in very similar positive benefits to the application of spin‐coated treatments demonstrating the promise of the spray‐passivation methodology. It is shown that devices created in this manner demonstrate PCEs of up to 21.0% (19.4% stabilized), representing the highest reported efficiency for one‐step, spray‐coated methylammonium‐free PSCs. This work represents the first demonstration of a spray‐coated surface passivation treatment that is compatible with high‐throughput, roll‐to‐roll processing.