The grain size and heavy metals (Cu, Pb, Zn, Cr, Cd, and Hg) in 42 seafloor and eight river sediments from the Kompong Som Bay of Cambodia were analyzed to identify sediment characteristics and environmental quality. The high-value areas for all the metals were mainly distributed in the northern and central parts of the bay, occupied by fine-grained sediments. There was slightly to moderately severe enrichment of Pb, Cd, and Hg in the southern and western coastal areas of Sihanoukville. Although the current ecological environment risk was relatively low, the coastal areas of Sihanoukville had low to moderate pollution of Pb, Cd, and Hg, which were closely related to pollutant emissions from the industrial production from power plants and oil refineries. It is recommended to establish a normalized monitoring mechanism and control the discharge of coastal pollutants to ensure the sustainable development of the coastal environment.