A comprehensive study and preservation of the plant world is one of the main challenges facing bo-tanists. Cyanoprokaryota (Cyanophyta, Cyanobacteria) also known as blue-green algae is the oldest group of prokaryote organisms involved in photosynthesis ha-ving a great impact on the global ecosystem. Three billion years ago their photosynthesis caused the “oxygen revolution” changing both the Earth atmosphere and its biota. This group consists of heterogeneous pool of photosynthetic prokaryotes and covers 2000 species belonging to several hundred genera represented by unicellular, colonial, filamentous or branched-filamentous forms. Cyanoprokaryota is widely distributed in aquatic to terrestrial habitats, in extreme environments such as hot springs, hypersaline ecosystems, deserts and polar regions. The Caspian Sea is a unique, reservoir with brackish water isolated from the world ocean, extended along meridian and located in the junction of Europe and Asia. The shores of Azerbaijan, in the length of 955 km, are washed by the waters of the Middle and South Caspian. The salinity of the water ranges between 12-13.4%, with the exception of the estuarine area of the Kura River, where the waters of the South Caspian periodically subjects to desalination. Blue-green algae (Cyanoprokaryota) are an integral component of the aquatic ecosystems of Caspian and their role is diverse. So blue-green algae play an important role in the sedimentation of the Caspian, forming an oil-like compound sapropel, which, it is not unlikely that is a precursor of oil in the Caspian Sea. Therefore, like its other components, blue-green algae need careful study and protection. Provided data are based on the results of long-term study of species diversity of Cyanoprokaryota in the Caspian Sea (including the Azerbaijani sector) from the 70s of the last century to the present. As a result, information was expanded on the species diversity, ecology and distribution of blue-green algae in the Azerbaijani sector of the Caspian Sea. The paper presents generalized literature and original data on taxonomic structure and species diversity of Cyanoprokaryota Azerbaijani sector of the Caspian Sea. In total, 93 species of cyanophytes belonging to class Cyanophyceae, three subclasses, five orders, 20 families and 44 genera are known for the studied area. Orders Synechococcales (7 families, 15 genera and 27 species) and Oscillatoriales (3 families, 13 genera and 35 species) lead in taxonomic and species diversity; together they incorporate 66.7% of the species revealed. Leading families are Oscillatoriaceae (21.8%) and Microcoleaceae (12.9%). Genera Phormidium Kützing ex Gomont (9 species), Oscillatoria Vaucher ex Gomont and Chroococcus Nägeli (6 species each), Leptolyngbya Anagnostidis & Komárek, Lyngbya C. Agardh ex Gomont, Merismopedia Meyen (5 species each) and Spirulina Turpin ex Gomont (4) lead in species diversity incorporating 43% of species found; 26 genera are represented by one species. Algoflora of the Azerbaijani sector of the Caspian Sea is also enriched with 55 species of blue-green algae, of which 25 species became an addition for the Caspian Sea as a whole.