Grain-size distribution dataset of lagoonal and riverine coastal placer deposits along the southeastern coast of Sri Lanka

• The data pertains to the unexplored areas of heavy mineral concentration along the lagoonal and riverine beaches of the southeastern coast of Sri Lanka. • The study focuses on the southwestern coastline edge of the Bay of Bengal, examining the variations in coastline GSD. • Vijayan Complex, which is the basement and sediment source of study area, is related to the evolution of an island arc, juvenile crust of Grenvillian age, Lützow-Holm Bay region in East Antarctica. • The data show the GSD variations between followings: ilmenite, garnet placer deposits and siliciclastic sediment; crescent-shaped bays and straight beach sediment; numerous fluvial outlets and their gradual sinking distance increment on beach; and alongshore (~70 km) beach sediments. • The data can be reused for the studies of alongshore and fluvial sediment dynamics, depositional and erosion processes, engineering applications, Precambrian host rock, renewable heavy mineral, Bengal Bay sediment. • The data can be beneficial to heavy mineral industries, sedimentologists, researchers, and civil engineers.