Knowledge of the mechanism of formation, orientation, and location of phases inside thin perovskite films is essential to optimize their optoelectronic properties. Among the most promising, low toxicity, lead‐free perovskites, the tin‐based ones are receiving much attention. Here, an extensive in situ and ex situ structural study is performed on the mechanism of crystallization from solution of 3D formamidinium tin iodide (FASnI3), 2D phenylethylammonium tin iodide (PEA2SnI4), and hybrid PEA2FAn−1SnnI3n+1 Ruddlesden–Popper perovskites. Addition of small amounts of low‐dimensional component promotes oriented 3D‐like crystallite growth in the top part of the film, together with an aligned quasi‐2D bottom‐rich phase. The sporadic bulk nucleation occurring in the pure 3D system is negligible in the pure 2D and in the hybrid systems with sufficiently high PEA content, where only surface crystallization occurs. Moreover, tin‐based perovskites form through a direct conversion of a disordered precursor phase without forming ordered solvated intermediates and thus without the need of thermal annealing steps. The findings are used to explain the device performances over a wide range of composition and shed light onto the mechanism of the formation of one of the most promising Sn‐based perovskites, providing opportunities to further improve the performances of these interesting Pb‐free materials. The structure and formation mechanism of spin‐coated films of lead‐free Sn‐based Ruddlesden–Popper (Sn‐RDP) perovskites are unveiled by combining results from in situ grazing incidence wide‐angle X‐ray scattering measurements and other extensive ex situ characterization methods. The formation of films with oriented Sn‐RDP crystallites is the result of bulk crystallization suppression induced by the presence of the 2D component (PEA+) during the drying process.