Hybrid lead halide perovskites have emerged as an important class of optoelectronic materials. A creative choice of "A"-site organic cations produces hybrid perovskites with reduced dimensionality and intriguing light−matter interactions. Nonlinear optical effects are expected to be stronger in one-dimensional (1D) lead halides compared to those in their 2D or 3D analogues due to greater quantum and dielectric confinements. We performed an extensive probe of the third harmonic generation (THG) properties in 1D pyridinium lead iodide single crystals for the first time. An efficient THG with a high laser-induced damage threshold is the highlight of this system. THG is selectively enhanced for excitation at an optical communication wavelength (1.5 μm) corresponding to band gap resonance. A strong exciton−phonon interaction results in highly Stokes-shifted self-trapped excitonic emission in the 5−300 K temperature range. An interplay between anharmonicity and dynamic disorder dictates the emission properties, further complicated by an isostructural phase transition at 170 K.

Low‐dimensional lead‐halide perovskites show wide structural and compositional diversity. Multiple functionalities can be introduced in these materials for diverse linear and nonlinear optical applications. Nonlinear optical (NLO) processes depend on material polarizability, crystal symmetry, and processibility. The flexibility of choosing organic cations in lead‐halide perovskites provides an opportunity for enhanced NLO responses by enhancing the overall system polarizability. A highly polarizable diisopropylammonium [Dipa; ((CH3)2CH)2NH2+] is used as an A‐site cation to synthesize one‐dimensional (1D) lead‐iodide (DipaPbI3) perovskite single crystals and the third‐harmonic generation (THG) in the excitation wavelength range of 1200–1600 nm is explored. DipaPbI3 single crystals exhibit exceptionally efficient resonance‐enhanced THG, amazing optical stability, and a high polarization ratio due to enhanced bulk‐polarization. The THG efficiency (χ(3) ≈ 4 × 10−18 m2 V−2) and laser‐induced damage threshold (LIDT, 144 GW cm−2) are an order of magnitude higher than the reported perovskites explored as THG materials. Density functional theory calculations show a significantly higher polarizability for Dipa cation, asserting the influence of the cation polarizability on the NLO efficiency of the perovskite. The stability of these single crystals in ambient conditions for an extended period adds to their potential for commercial application in the near future.