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Mangrove ecosystems support diverse macroalgal assemblages, which are primary producers. Community composition, percentage cover of macroalgae, and physicochemical parameters of seawater were studied at two mangrove areas in the Red Sea, Egypt, from November 2020 to October 2021. In addition, we sought to study changes in the relative densities of macroalgal species of the Red Sea in the two studied areas by using a comparison of our studying data (2020–2021) with previous studies carried out in the same locations in 1997–1998. In order to characterize the spatial variations in algal composition, three 20 m permanent line transects and three 1-m2 quadrates were established at each site. Throughout the duration of the study, the physicochemical parameters varied significantly (p < 0.05) among the seasons and were slightly different between the two sites. Twenty-four species of macroalgae were collected and identified from the studied sites. Approximately 29.2% of these algae (7 species) belong to the Chlorophyta, 45.8% (11 species) to the Ochrophyta (class: Phaeophyceae), and 25% (6 species) to the Rhodophyta. Phaeophyceae species had the highest diversity and richness values at both sites. Comparing the composition of the community during the years 2020–2021 and 1997–1998 yielded no statistically significant differences (p > 0.05). Mangrove trees support a high macroalgae cover, resistant to most environmental circumstances. The macroalgal assemblages' stability in the two studied areas over the two decades shows the health and quality of the Red Sea waters. [ABSTRACT FROM AUTHOR]