The warming ocean is expected to be more phosphorus (P) limited due to increasing stratification. P is a major limiting nutrient of marine diazotrophs, while the interactive effect of temperature elevation and P limitation on marine unicellular diazotrophs is unknown. Here, we examined the physiology of a major unicellular diazotroph, Crocosphaera watsonii, grown under P-limited and P-replete conditions at 25°C, 28°C, and 31°C. Growth, N₂, and CO₂ fixation rates of C. watsonii increased with temperature under P limitation, and growth rates were similar between P-limited and P-replete treatments at 31°C. At high temperature, the P use efficiencies for N₂ and CO₂ fixation under P limitation were more than twice higher than under P-replete conditions. Expression of genes involved in P acquisition, intracellular recycling, and substitution in C. watsonii was upregulated at higher temperature under P limitation. These results suggest that P limitation in C. watsonii was relieved with elevated temperature through various temperature-dependent economic strategies on P metabolism. Through meta-analysis of a field data set using general additive model, we found that C. watsonii abundance was correlated mainly with temperature and phosphate, and predicted to increase significantly with further warming.