LaMg6Ga6S16 is a chemically stable divalent lanthanide chalcogenide synthesized by constructing an octahedral crystal field through coupling tetrahedral single/double chains. The divalent La2+ cations were identified using X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, and electron paramagnetic resonance, while the stability was confirmed by differential thermal scanning, in-situ variable-temperature powder X-ray diffraction, and solid-state reactions. LaMg6Ga6S16 exhibits ultrabroad-band green emission at 500 nm, the first instance of La(II)-based compounds with luminescent properties. It also shows second-harmonic generation (SHG) activity due to its non-centrosymmetric space group P-6, with a comparable SHG response to classical AgGaS2. LaMg6Ga6S16 has a wide band gap (3.0 eV) and a high laser-induced damage threshold (5×AgGaS2), making it a promising nonlinear optical material. The synthesis, structure, and luminescent and NLO properties of LaMg6Ga6S16 are reported.LaMg6Ga6S16 is a chemically stable divalent lanthanide chalcogenide synthesized by constructing an octahedral crystal field through coupling tetrahedral single/double chains. The divalent La2+ cations were identified using X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, and electron paramagnetic resonance, while the stability was confirmed by differential thermal scanning, in-situ variable-temperature powder X-ray diffraction, and solid-state reactions. LaMg6Ga6S16 exhibits ultrabroad-band green emission at 500 nm, the first instance of La(II)-based compounds with luminescent properties. It also shows second-harmonic generation (SHG) activity due to its non-centrosymmetric space group P-6, with a comparable SHG response to classical AgGaS2. LaMg6Ga6S16 has a wide band gap (3.0 eV) and a high laser-induced damage threshold (5×AgGaS2), making it a promising nonlinear optical material. The synthesis, structure, and luminescent and NLO properties of LaMg6Ga6S16 are reported.