Environmental degradation caused by industrial development and urbanization poses significant threats to aquatic ecosystems, emphasizing the urgent need for effective monitoring tools. Diatoms, highly adaptable microalgae, serve as bioindicators of environmental change due to their sensitivity to variations in water quality. This study aimed to investigate the diversity of epipelic diatoms in sediment layers of intertidal waters in Dumai City, Riau, to assess their potential as indicators of aquatic environmental health. This research was conducted from May to October 2022 in the intertidal waters of Dumai City, Riau. The method used was the survey method, and purposive sampling was conducted at four stations, namely Dumai River Estuary (ST1), Masjid River Estuary (ST2), Bangsal Aceh Industrial Area (ST3), and Koneng Beach (ST4). Sediment samples were collected from ten depth intervals (0–5cm to 46–50cm) to evaluate species distribution, diatom abundance, and ecological indices, including diversity (H'), dominance (D), and uniformity (E). Results revealed 25 diatom species, with the highest abundance in the 0–5cm sediment layer, particularly at ST2, which exhibited elevated nitrate and phosphate levels. The diversity index (2.06–2.98) indicated balanced biotic communities, while dominance and uniformity indices suggested an absence of competitive exclusion and a stable ecological state. These findings highlight the value of diatom diversity as a bioindicator for monitoring nutrient pollution and environmental health. Water quality in Dumai City is still in the tolerable range for diatom abundance and survival. This study highlights the potential of epipelic diatoms as biological indicators for assessing the impact of anthropogenic activities and environmental changes on Dumai's aquatic ecosystems.