The incorporation of large organic cations in between “perovskite slabs” to form two-dimensional (2D) hybrid perovskites has been reported to mitigate the degradation against the environmental agents of the three-dimensional (3D) perovskites widely employed in optoelectronic devices. The 2D hybrid perovskites, in particular the Ruddlesden-Popper (RP) phase, exhibit excellent optoelectronic properties with a wide flexibility in the type of large organic cations that can be employed. However, the small organic cations inserted in the octahedral voids have been limited so far to those three fulfilling the Goldschmidt tolerance factor (t) despite the relaxed structure of the 2D RP perovskites that may open the way to the insertion of other cations. In this study, the incorporation into the octahedral sites of the “perovskite slabs” of a large ammonium cation neglecting the tolerance factor (Guanidinium, Gua) has been explored for the first time in 2D RP perovskites. Thus, the methylammonium (MA) cation in the PEA₂MA₂Pb₃I₁₀ perovskite (PEA = phenylethylammonium) has been gradually substituted by the Gua cation to synthesize thin films of the mixed cation PEA₂(MA_1-xGua_x)₂Pb₃I₁₀ perovskite. X-ray Diffraction (XRD) and GrazingIncidence Wide-Angle X-ray Scattering (GIWAXS) measurements have revealed a regular expansion of the unit cell when increasing the Gua content up to 90% proving the sequential insertion into the lattice of the Gua having a larger ionic radius than that of the MA cation. Furthermore, the preferential orientation of the PEA₂MA₂Pb₃I₁₀ perovskite films with the (hk0) planes parallel to the substrate is maintained up to a limit value of 60% Gua content. Scanning Electron Microscopy (SEM) has also displayed a densely packed grain morphology that is not modified due to the addition of Gua. Importantly, the combined analysis of the absorption and photoluminescence (PL) spectra have revealed a change in the distribution of the n-members of the 2D RP perovskites towards phases with low n values upon increasing the Gua content. In particular, a sudden change is observed at 30% Gua content which is related to the impossibility of the phases with high n values to incorporate more than 25% Gua in their structure. Thus, the addition of a large organic cation that substitutes the small MA cation plays a key role to control the distribution of n-members in the 2D RP perovskite films. With this strategy, it is possible to fine tune the position and shape of the absorption and PL spectra and to significantly increase 25 times the PL intensity of the films. Theoretical calculations on the 2D RP perovskites have proven that the incorporation of the Gua cation to the low dimensional perovskite (n = 3) is energetically more favorable with respect to the insertion in the 3D material. Finally, the stability against the environmental factors of the 2D RP perovskite films without encapsulation has been significantly improved even at low percentages of Gua.