The objective of this study is to investigate the possibility of using seawater on a clay type of soil instead of using tap or fresh water in road constructions fill works. When compared to the samples that were prepared using tap water, it was seen that the samples prepared with seawater showed no change in consistency limit, but the soil's unconfined compressive strength value increased to 831 kPa from 300 kPa in 28 days time. The soil's soaked CBR values also increased by 7 times to a value of 27, while the swelling potential dropped to zero. These changes in soil properties may lead to transforming existing unsuitable in-situ materials into suitable fill materials. With the improvement in the soil with seawater, the existing soil can easily be used as a suitable fill material in the lower layers of the filling in road construction works. These changes will reduce consumption of natural waters as well as the use of quarried stone and the transportation of filling material, thereby making several important contributions to the protection of natural resources.