Abstract. Macrophyte algae and microalgae of the southern shores of the Caspian Sea (Republic of Azerbaijan, Lankaran district, and Gizilaghaj National Park) were studied from March to December 2022. A total of 29 algae species were identified, 11 of which were located in Gizilaghaj National Park. These species belong to Chlorophyta (9), Bacillariophyta (7), Cyanobacteria (6), Charophyta (4), and Rhodophyta (3). Oscillatoriaceae, Spirogyraceae, Cladophoraceae, and Naviculaceae were the leading families. Lyngbya and Spirogyra dominated the upper littoral zone, while Cladophora dominated the sublittoral zone. Twenty-seven species were recorded as new to this region, with Cladophora albida, C. ruchingeri, Ulva torta, Colaconema elegans, Corallina officinalis, and Jania virgata being recognized as both new and rare species. Seasonal biodiversity peaked in July. Ecological analysis highlighted pollution tolerance levels based on the Saprobic system. These findings emphasize the ecological importance of the region and the need for further studies to conserve its biodiversity.