jats:titleAbstract</jats:title>jats:pReduced‐dimensional perovskites (RDPs) have advanced perovskite optoelectronic devices due to their tunable energy landscape, structure, and orientation. However, the origin of structural and photophysical property changes when moving from low‐dimensional to high‐dimensional RDPs remains to be understood. This study systematically reveals structural and photophysical properties of slot‐die‐coated Dion‐Jacobson (DJ) and Ruddlesden‐Popper (RP) RDPs with different dimensionalities. RP RDPs with lower dimensionality (jats:italicn </jats:italic>= 2) exhibit a dominant jats:italicn </jats:italic>= 2 phase, preferential out‐of‐plane orientation, and longer charge carrier lifetime compared with DJ RDPs. In addition, the formation kinetics of RDPs with higher dimensionality (jats:italicn </jats:italic>= 4) are unraveled by in situ X‐ray scattering, showing the favorable formation of the lower‐jats:italicn</jats:italic> phase in RP RDPs. The formation of these lower‐jats:italicn</jats:italic> phases is thermodynamically and stoichiometrically favored, while these phases are likely in the form of an “intermediate phase” which bridges the 3D‐like and lower‐jats:italicn</jats:italic> phases in DJ RDPs. DJ RDPs with higher dimensionality demonstrate comparable phase purity, preferential orientation, spatially vertical phase homogeneity, and longer charge carrier lifetime. As such, DJ‐based perovskite solar cells (PSCs) (jats:italicn </jats:italic>= 4) demonstrate better photostability under operational conditions than RP‐based PSCs. Thus, the work paves the way for the utilization of RDPs to upscale PSCs.</jats:p>