Liquid limit tests were conducted on Mexico City clay samples prepared at different pore water salinities. The results demonstrate that the liquid limit decreases almost linearly from 340% to 275% as salinity decreases from 54 to 3 g/L, and then exhibits a sharp decrease for salinities below 3 g/L. X-ray analyses indicated that smectite is the main clay mineral in the clay, and a sediment volume test on clay fractions confirmed that the smectite is of a high-swelling type. The contents of Al, Si, and Fe extracted with oxalate were 26.9, 37.8, and 246 g/kg, respectively. Despite the fact that the smectite in the Mexico clay is of a high-swelling type, the change in the liquid limit due to pore water salinity was opposite to that for bentonite and paddy soil composed of high-swelling smectite but was similar to that for marine clays composed of low-swelling clay minerals. This characteristic was ascribed to a dominant role of poorly crystalline aluminosilicates and iron (ferrihydrite) with low-swelling character over the high-swelling smectite in determining the liquid limit of the Mexico clay as the pore water salinity is changed.