Inorganic halide perovskites possessing noncentrosymmetric (NCS) structures are highly desirable for anomalous photovoltaic, ferroelectric, and nonlinear optical (NLO) applications. In this work, we investigate the Ruddlesden-Popper (RP) germanium (Ge) halide perovskites ${\mathrm{Cs}}_2\mathrm{Ge}{X}_4$ ($X=\mathrm{Cl}$, Br, I) to identify low-energy NCS phases and evaluate their NLO properties via first-principles calculations. Starting from the centrosymmetric $I4$/mmm parent phase, we systematically introduce 40 symmetry-allowed structural distortion modes, including 37 octahedral rotation/tilt modes and 3 polar displacement modes, to generate a set of low-symmetry configurations. Our results reveal that NCS configurations are energetically favored in ${\mathrm{Cs}}_2\mathrm{Ge}{X}_4$ relative to their centrosymmetric counterparts, in contrast to analogous lead-based RP perovskites, which tend to stabilize centrosymmetric structures. The lowest energy NCS structure is identified as the polar $\mathit{Pc}$ phase. Symmetry-mode analysis indicates that the polar ${\mathrm{\Gamma }}_5^{-}$ distortion in these Ge-based systems originates from the stereochemically active lone-pair electrons of $\mathrm{G}{\mathrm{e}}^{2+}$, which drives inversion-symmetry breaking and stabilizes the NCS structure. Furthermore, we compute the second-order nonlinear susceptibility and second-harmonic-generation response of $\mathit{Pc}$-phase ${\mathrm{Cs}}_2\mathrm{Ge}{X}_4$, confirming its promising NLO performance. This study provides mechanistic insight into lone-pair-induced noncentrosymmetry in layered germanium halide perovskites and underscores their potential for advanced nonlinear optical applications.

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• Second order nonlinear optical processes
• Ruddlesden-Popper compounds
• Density functional calculations
• Optical second-harmonic generation