AbstractPhosphoglycerate dehydrogenase (PHGDH) is the first enzyme in de novo Ser biosynthesis. Numerous metabolic pathways rely on Ser as a precursor, most notably one‐carbon metabolism, glutathione biosynthesis, and de novo nucleotide biosynthesis. To facilitate proliferation, many cancer cells shunt glycolytic flux through this pathway, placing PHGDH as a metabolic liability and feasible therapeutic target for the treatment of cancer. Herein, we demonstrate the post‐translational modification (PTM) of PHGDH by lactoylLys. These PTMs are generated through a non‐enzymatic acyl transfer from the glyoxalase cycle intermediate, lactoylglutathione (LGSH). Knockout of the primary LGSH regulatory enzyme, glyoxalase 2 (GLO2), results in increased LGSH and resulting lactoylLys modification of PHGDH. These PTMs reduce enzymatic activity, resulting in a marked reduction in intracellular Ser. Using stable isotope tracing, we demonstrate reduced flux through the de novo Ser biosynthetic pathway. Collectively, these data identify PHGDH as a target for modification by lactoylLys, resulting in reduced enzymatic activity and reduced intracellular Ser.