Engineering properties of an extremely soft marine clay (locally known as Chikoko) were determined using tap water and compared with its properties when Oyorokoto sea water is used. Test results show considerable improvement in its properties when sea water is used. There was a reduction in plasticity index, free swell value, swell pressure and in the maximum dry unit weight, while the unsoaked CBR and optimum moisture content increased. There was however no significant improvement in the unconfined compressive strength (UCS).