The study presents quantitative niche models for the marine cyanobacteria Prochlorococcus and Synechococcus, assessing their current and future global distributions. These models, developed using over 35,000 observations from various ocean regions, consider temperature and photosynthetically active radiation (PAR) as key environmental factors. Prochlorococcus is most abundant in warm, oligotrophic gyres of the Indian and western Pacific Oceans, while Synechococcus has a broader distribution, including polar and high-nutrient waters. The global annual mean abundances are 2.9 × 10²⁷ and 7.0 × 10²⁶ cells for Prochlorococcus and Synechococcus, respectively. Under future climate scenarios, Prochlorococcus and Synechococcus are projected to increase by 29% and 14%, respectively, with uneven geographic distribution. The models suggest complex changes in ocean microbial communities due to climate change, potentially impacting marine ecosystems and biogeochemical cycles. Prochlorococcus and Synechococcus are major contributors to marine primary production, and their abundance changes could significantly affect oceanic ecosystems. The study highlights the importance of temperature and PAR in determining the distributions of these cyanobacteria, with Prochlorococcus showing a steeper response to temperature between 10–15°C and Synechococcus having a lower temperature threshold. Nutrient availability has limited influence on their distributions, as both species can thrive in a wide range of nutrient conditions. The models provide a quantitative basis for understanding future changes in these cyanobacteria's distributions and their ecological impacts.The study presents quantitative niche models for the marine cyanobacteria Prochlorococcus and Synechococcus, assessing their current and future global distributions. These models, developed using over 35,000 observations from various ocean regions, consider temperature and photosynthetically active radiation (PAR) as key environmental factors. Prochlorococcus is most abundant in warm, oligotrophic gyres of the Indian and western Pacific Oceans, while Synechococcus has a broader distribution, including polar and high-nutrient waters. The global annual mean abundances are 2.9 × 10²⁷ and 7.0 × 10²⁶ cells for Prochlorococcus and Synechococcus, respectively. Under future climate scenarios, Prochlorococcus and Synechococcus are projected to increase by 29% and 14%, respectively, with uneven geographic distribution. The models suggest complex changes in ocean microbial communities due to climate change, potentially impacting marine ecosystems and biogeochemical cycles. Prochlorococcus and Synechococcus are major contributors to marine primary production, and their abundance changes could significantly affect oceanic ecosystems. The study highlights the importance of temperature and PAR in determining the distributions of these cyanobacteria, with Prochlorococcus showing a steeper response to temperature between 10–15°C and Synechococcus having a lower temperature threshold. Nutrient availability has limited influence on their distributions, as both species can thrive in a wide range of nutrient conditions. The models provide a quantitative basis for understanding future changes in these cyanobacteria's distributions and their ecological impacts.