Abstract: We report a mechanistic study of excitonic photoluminescence in predesigned hybrid organic–inorganic perovskite (HOIP) systems, i.e., (DMAEA)Pb₂I₆, (DMAPA)PbI₄, (DEAEA)Pb₂I₆, and (DEAPA)₄Pb₅I₁₈, featuring targeted regulation of organic cations. Starting from the prototype DMAEA (i.e., 2-N,N-dimethylamino-1-ethylamine) for (DMAEA)Pb₂I₆, the other three HOIPs differ only in the extensions with CH₂ group(s) at the “head” or/and “tail” of DMAEA that is an “alkylated ammonia”. Their crystal structures are constructed and structural distortions are evaluated. The steady-state/transient absorption and emission spectroscopic characterizations, combined with the band-structure calculations, are conducted. The two different photoluminescence (PL) mechanisms are identified, i.e., PL emissions dominated by free excitons for (DMAPA)PbI₄ and by self-trapped excitons for (DMAEA)Pb₂I₆, (DEAEA)Pb₂I₆, and (DEAPA)₄Pb₅I₁₈. The self-trapped excitonic effect involved in the latter three HOIPs is quantitatively analyzed. This work would be of guiding value for the design of HOIP systems based on organic-cation engineering, beneficial for the pertinent performance optimization in light-emitting applications.