AbstractReducing the dimensionality of three-dimensional hybrid metal halide perovskites can improve their optoelectronic properties. Here, we show that the third-order optical nonlinearity, n₂, of hybrid lead iodide perovskites is enhanced in the two-dimensional Ruddlesden-Popper series, (CH₃(CH₂)₃NH₃)₂(CH₃NH₃) _n-1Pb _n I_3n+1 (n = 1–4), where the layer number (n) is engineered for bandgap tuning from E_g = 1.60 eV (n = ∞; bulk) to 2.40 eV (n = 1). Despite the unfavorable relation, ${n_2} ∝ E_{\rm{g}}^{ - 4}$ n 2 ∝ E g - 4 , strong quantum confinement causes these two-dimensional perovskites to exhibit four times stronger third harmonic generation at mid-infrared when compared with the three-dimensional counterpart, (CH₃NH₃)PbI₃. Surprisingly, however, the impact of dimensional reduction on two-photon absorption, which is the Kramers-Kronig conjugate of n₂, is rather insignificant as demonstrated by broadband two-photon spectroscopy. The concomitant increase of bandgap and optical nonlinearity is truly remarkable in these novel perovskites, where the former increases the laser-induced damage threshold for high-power nonlinear optical applications.