Abstract: To investigate the species diversity and spatial distribution characteristics of macroalgae in the Xisha Islands. In this study, we identified the species composition of macroalgae in spring and summer around 11 typical islands and reefs of the Xisha Islands, namely Yongxing Island, North Island, Zhaoshu Island, Jinqing Island, Shiyu, Yinyu, Quanfu Island, Ganquan Island, Chenhang Island, Shanhu Island, and Jinyin Island, using a circum-insular diving sampling method. The Shannon diversity index, Margalef richness index, and Pielou evenness index of macroalgae were analyzed for each island. A total of 100 species of macroalgae were collected in this survey, belonging to 57 genera, 29 families, 18 orders, 3 classes, and 3 phyla. Dictyosphaeria was the most widely distributed species in this survey; Galaxaura, Halimeda incrassata, and H. macroloba were dominant or important species in most islands and reefs. Based on a comprehensive analysis of species number and diversity indices, Yongxing Island had the highest number of species and the highest Shannon diversity index; Zhaoshu Island had a relatively low number of species and the lowest Margalef index; Ganquan Island had the lowest number of species but the highest Margalef index; Quanfu Island had a relatively rich species list but exhibited the lowest Shannon diversity index and Pielou evenness index. Combining the results of this study with literature records, a total of 344 species of macroalgae are known in the Xisha Islands, belonging to 124 genera, 41 families, 21 orders, 3 classes, and 3 phyla. Among them, Rhodophyta is the most abundant, containing 194 species in 81 genera, 21 families, 9 orders, and 1 class, accounting for 56.40% of the total species; Chlorophyta follows, with 90 species in 27 genera, 13 families, 7 orders, and 1 class (26.16%); Ochrophyta (Phaeophyceae) includes 60 species in 16 genera, 7 families, 5 orders, and 1 class (17.44%). Through this background investigation and analysis of macroalgae resources around 11 islands and reefs of the Xisha Islands in spring and summer, this study systematically clarifies the species composition and floristic characteristics of macroalgae in the region during spring and summer, providing reference data for the protection, restoration, and sustainable management of the Xisha Islands ecosystem.