Marine eukaryotic phytoplankton adapt to low phosphorus (P) in the oceans through a variety of step‐wise mechanisms including lipid substitution and decreased nucleic acid content. Here, we examined the impact of low P concentrations on intracellular metabolites whose abundances can be quickly adjusted by cellular regulation within laboratory cultures of three model phytoplankton and in field samples from the Atlantic and Pacific Oceans. We quantified the relative abundances of monophosphate nucleotides and their corresponding nucleosides, using a combination of targeted and untargeted metabolomics methods. Under P‐deficient conditions, we observed a marked decrease in adenosine 5′‐monophosphate (AMP) with a concomitant increase in adenosine. This shift occurred within all detected pairs of monophosphate nucleotides and nucleosides, and was consistent with previous work showing transcriptional changes in nucleotide synthesis and salvage under P‐deficient conditions for model eukaryotes. In the field, we observed AMP‐to‐adenosine ratios that were similar to those in laboratory culture under P‐deficient conditions.