Algae are important flora in mangrove ecosystems. Algae can be divided into three ecological groups, namely phytoplankton, benthic microalgae, and macroalgae in mangrove ecosystems, which play important roles in organic carbon production and nutrient cycle. Despite the importance of algae for ecosystem function, studies of mangrove ecosystems have focused on higher plants and animals, with few studies of algae. Due to their abundance in mangrove ecosystems, studies of algae can broaden our understanding about the structure and function of mangrove ecosystems. In this review, we first briefly introduce algal groups and their ecological importance in mangrove ecosystems. Then, we emphasize species composition and geographical distribution of phytoplankton, benthic diatoms and macroalgae, and their importance in key ecological processes such as primary production, water pollution, element cycle, and carbon stock dynamics in mangrove ecosystem. The researches have showed that the species number of phytoplankton and benthic diatoms in mangroves varied from dozens to hundreds, and diatoms are dominant both in species composition and abundance, which are important primary producer, animal food, and pollution indicator. Macroalgae are mainly composed of red algae, green algae, brown algae and blue-green algae. Green algae are dominant in species richness whereas red algae are abundant in quantity. Algae contribute significantly to the carbon pool by sequestering and cycling carbon. We propose that future studies should focus on algal diversity and its role in ecological processes in mangrove ecosystems. Furthermore, we suggest that studies of algae should be part of comprehensive investigations on long-term ecosystem dynamics. The influence of continental runoff and tidal patterns on algal diversity and blue carbon dynamics in mangrove areas also deserve more attention, on account of being directly related to nutrient replenishment and dynamics. Since algal diversity in mangrove sediment is influenced by physico-chemical and biological condition of the system, it could be used as an indicator of climate change and effects of anthropogenic activity on mangrove ecosystems.