This study dealt with analyzing of naturally occurring 40K radioisotope in beach sand and seawater samples of Sarımsaklı beach at Aegean coastal region of Turkey and its contribution to natural radioactivity. By using gamma spectroscopy the mean radioactivity concentration was found as 1093.00 ± 115.07 Bq kg-1 for the beach sand and as 14.08 ± 3.50 Bq kg-1 for the seawater. The average external effective dose rate of beach sand was determined in air at 1 m above the ground as 45.58 ± 4.80 nGy h-1.  Keywords: Aegean Sea, Gamma Spectroscopy, 40K; Beach Sand, Seawater, Turkey