Abstract <p>This review is devoted to cyanobacterial metabolites with allelopathic activity. The allelopathic properties of cyanobacteria have been studied relatively recently, and many aspects of this problem remain controversial. The chemical structure of allelochemicals produced by cyanobacteria is diverse (alkaloids, phenolic compounds, organic acids, cyclic peptides, fatty acids, etc.), and the spectrum of biological action is extremely wide and covers almost all aquatic inhabitants. Allelopathy is considered a strategy for regulating phytoplankton communities, helping maintain species diversity in aquatic ecosystems. Cyanotoxins, including microcystins, exhibit high allelopathic activity towards aquatic organisms. In addition to the effect of cyanobacterial allelochemicals on phytoplankton, they exhibit antifungal, antibacterial, antiviral, antioxidant, anti-inflammatory, and antitumor activity, which indicates their potential for application in agriculture and pharmacology. The ecological role of allelochemicals, the mechanisms of action, and the influence of environmental factors are discussed.</p>