The proteinogenic amino acid L-serine is a precursor for various essential biomolecules in all organisms. 3-Phosphoglycerate dehydrogenase (PGDH) is the first committed enzyme of the phosphorylated pathway of L-serine biosynthesis, and is regulated by negative feedback from L-serine in bacteria and plants. In the present study, two Arabidopsis PGDH isoforms were inhibited by L-serine but were activated by L-amino acids such as L-homocysteine in vitro. Activation and inhibition by these amino acids was cooperative, suggesting an allosteric mechanism. Moreover, the half maximal effective concentration of L-homocysteine was 2 orders of magnitude lower than that of L-serine, suggesting greater regulatory potency. These are the first data to show that PGDH is activated by various biomolecules and indicate that serine biosynthesis is regulated by multiple pathways.