Twenty new alkali rare earth thiosilicates and thiogermanates with the general formula ALnTS(4) (A = alkali metal, Ln = lanthanide, and T = Si, Ge) were grown as X-ray diffraction-quality single crystals from molten alkali chloride fluxes. These include KNdSiS4, KPrSiS4, RbLnSiS(4) (Ln = Ce, Pr, Nd, Gd, Tb, Dy, and Ho), RbLaGeS4, CsLnSiS(4) (Ln = La, Pr, and Nd), and CsLnGeS(4) (La, Ce, Pr, Nd, Eu, Gd, and Tb). Herein, we discuss the use of a molten chloride flux growth approach for the preparation of the title compounds and their structure determination via single-crystal X-ray diffraction. In addition, we comment on the magnetic properties of RbNdSiS4, CsNdSiS4, CsNdGeS4, and CsGdGeS4, which were found to be paramagnetic for T = 2-300 K and exhibited negative Weiss temperatures with no obvious antiferromagnetic transition down to 2 K. The optical properties of CsLaGeS4 and CsNdTS4 (T = Si, Ge) were measured by UV-vis spectroscopy. Second harmonic generation measurements performed on CsLaGeS4 confirmed the crystallization of the compound in the noncentrosymmetric orthorhombic space group, P2(1)2(1)2(1); CsLaGeS4 was found to be SHG-active with nearly half the intensity of alpha-SiO2 upon irradiation with a Nd:YAG 1064 nm laser, and a semiconductor exhibiting a band gap of 3.60 eV based on UV-vis diffuse reflectance measurements.