The salts formed in the l-cysteine–HI–H₂O system were studied. Three salts were obtained as single crystals, namely, anhydrous l-cysteinium iodide (l-CysH)I, l-cysteinium iodide monohydrate (l-CysH)I·H₂O and l-cysteinium···l-cysteine iodide hemihydrate (l-CysH···l-Cys)I·0.5H₂O, which is a new salt with dimeric cation. Crystal structures of all three salts were solved: anhydrous and hydrated iodides crystallize in the orthorhombic space group P2₁2₁2₁, whereas (l-CysH···l-Cys)I·0.5H₂O crystallizes in the monoclinic space group I2. The dimeric cation is formed by a very short O–H···O hydrogen bond with (O···O) of 2.436(3) Å. The crystal structures were compared with those of other crystalline l-cysteine halogenides l-CysH(l-CysH···l-Cys)F(F···HF), (l-CysH···l-Cys)Cl, (l-CysH···l-Cys)Br, (l-CysH)Br·H₂O (previously studied by us), and (l-CysH)Cl·H₂O (previously reported in literature). In addition to crystal structure analysis by X-ray diffraction techniques, infrared and Raman spectra were registered and discussed. Second harmonic generation characteristics by these iodides were measured for powders and compared with those reported for chlorides and bromides.