AbstractUnicellular photoautotrophic diazotrophs such as Crocosphaera spp. are ubiquitous in many oligotrophic and N‐limited oceans, as they can reduce N2 into bioavailable ammonia. The Mediterranean Sea is potentially an ideal environment for photoautotrophic diazotrophic activity, and yet N2‐fixation rates measured in the last two decades are typically very low and no diazotrophic blooms have been recorded in its offshore waters. Previous studies suggest that diazotrophs, as well as nondiazotrophic phytoplankton and heterotrophic bacteria, may be P‐limited, hence their low biomass and activity. Here, we amended surface seawater from six stations across a nutrient gradient in the Mediterranean Sea (east to west transect) with dissolved inorganic phosphorus (DIP), and with seawater‐acclimated inocula of Crocosphaera watsonii, a unicellular cyanobacterial diazotroph, to examine if DIP can stimulate diazotrophy. Our results demonstrate that C. watsonii are poor competitors for DIP relative to native nondiazotrophic heterotrophic microbial populations, especially in the ultraoligotrophic eastern Mediterranean basin, resulting in low N2‐fixation rates. Moreover, the results indicate that when the ambient DIP concentrations are < 35 nmol L−1, unicellular photoautotrophic diazotrophs such as C. watsonii will likely be outcompeted for this macronutrient in the Mediterranean Sea, whereas above 35 nmol L−1 diazotrophy can be stimulated. Our findings support the “bypass theory” stating that photoautotrophs may be outcompeted by heterotrophic bacteria for DIP in nutrient‐poor regions such as the Mediterranean Sea.