Single crystals of nonlinear optical material l-arginine acetate (LAA) (C6H14N4O2·CH3COOH), space group P21, were successfully grown for the first time by the temperature-lowering method and also by the slow evaporation method at constant temperature (30 °C) from its aqueous solution with pH at 6 and dimension of 21 × 15 × 3 mm3. Initially, solubility tests were carried out for four solvents such as water, water and methanol, water and ethanol, and water and acetone. Among the four solvents, the solubility of LAA was found to be the highest in water, so crystallization of LAA was done from its aqueous solution. Morphological analysis reveals that the crystal is a polyhedron with 16 developed faces with major face forms {100}, {001}, and {102} (pinacoids) parallel to the polar axis. As grown crystals were characterized by chemical analysis, density measurement, and X-ray powder diffraction studies. Infrared spectroscopy, thermogravimetric analysis, and differential thermal analysis measurements were performed to study the molecular vibration and thermal behavior of LAA crystals. Thermal analysis does not show any structural phase transition.