In the quest for alternative perovskite materials with reduced toxicity, tin (Sn) perovskites are emerging. They, however, suffer from inherent material instability and difficulty to control crystallization which leads to high defect density in the thin films. In this work, we demonstrate methylammonium chloride (MACl)‐assisted crystallization as a route to simultaneously improve stability and optoelectronic quality of quasi 2D/3D PEA_0.08FA_0.92SnI₃ perovskite. For an optimum value of 10% of this additive we find a 37% increase in power conversion efficiency. Notably, the MACl inclusion enhances ambient‐ and photo–stability of the perovskite films, as evidenced by temporal PL tracking of non‐encapsulated films in air. Understanding the effect of MACl addition in this system is of significant interest for the pursuit of more efficient and stable tin‐based devices. Our investigations show that MACl addition leads to a small shift in the optical band gap and enhanced grain size, indicating that the bulk crystal structure is affected. X‐ray photoelectron spectroscopy confirms the presence of Cl on the surface of the perovskite films, while no indication of MA⁺ is found. Intriguingly, UV photoelectron spectroscopy shows pronounced changes in the density of states. We show for the first time that the additive promotes the formation of low dimensional surface layer via the accumulation of PEA⁺ on the surface. The MACl additive also leads to reduction in ionization energy from 5.4 eV in the reference samples to 5.2 eV for the 10% MACl counterpart, which could facilitate efficient hole extraction in the devices. Overall, our work highlights a facile route to control the crystallization of Sn‐perovskites to simultaneously enhance their optoelectronic quality and ambient stability.This article is protected by copyright. All rights reserved.