The paper describes the taxonomic diversity of the Ericaria–Gongolaria phytocenosis (Ericaria crinita + Gongolaria barbata – Cladostephus spongiosus – Ellisolandia elongata), which is the key part of the coastal ecosystems of specially protected natural areas of Sevastopol. The material was collected according to a standard procedure in the waters of six protected sites at depths of 1–5 m in summers from 2016 to 2021 during the peak of the macrophyte growing season. When analyzing the samples, we considered the species composition of the phytocenosis, the duration of life cycle of macroalgae, and their distribution by tiers and depths. The phytocenosis structure was found to be characterized by a high phytodiversity. Totally, 34 species of macroalgae were recorded. Two of them were found in the first tier, seven species were included in the second tier, and 25 taxa were represented in the third tier. The greatest diversity was found in red (Rhodophyta) algae: their share accounted for 52% of the total number of species. The contribution of brown (Ochrophyta) and green (Chlorophyta) macroalgae was 2.2 times less (24% each). In terms of life cycle, perennial and annual algae predominated: their share was 44 and 38%, respectively. The largest contribution of perennial species (70%) was registered near Cape Aya and Cape Sarych. In general, Ericaria–Gongolaria phytocenosis of different specially protected natural areas was characterized by low floral similarity: the values of the Jacquard coefficient (Kj) ranged from 7.1 to 66.7% and the average value was 21%. The taxonomic diversity, full tier structure and the predominance of perennial species indicate the stability of the key phytocenosis in specially protected natural areas of Sevastopol and effectiveness of their environmental regime.