Serine starvation induces stress and p53-dependent metabolic remodeling in cancer cells. Cancer cells rely on specific metabolic adaptations to survive stress and support proliferation. The tumor suppressor p53 plays a key role in regulating various metabolic processes, including glycolysis, oxidative phosphorylation, glutaminolysis, and antioxidant responses. While p53 promotes apoptosis during DNA damage, it also supports cell survival during metabolic stress, which may contribute to both tumor suppression and non-cancer functions. This study shows that cancer cells rapidly utilize exogenous serine, and serine deprivation activates the serine synthesis pathway (SSP), reducing aerobic glycolysis and increasing flux to the TCA cycle. Transient p53-p21 activation and cell cycle arrest promote survival by channeling depleted serine to glutathione synthesis, maintaining antioxidant capacity. p53-deficient cells fail to respond to serine depletion, leading to oxidative stress, reduced viability, and impaired proliferation. In vivo models confirm that serine depletion can target p53-deficient tumors. Serine and glycine are essential nutrients for cancer cell proliferation, and their removal significantly reduces cell viability. p53-deficient cells show greater sensitivity to serine and glycine depletion than p53-proficient cells. Analysis of cell culture media shows rapid serine consumption by both p53-proficient and -deficient cells, while glycine uptake is low. Cancer cells avidly consume serine, which can be converted into methyl-THF and glycine via high SHMT expression. Unlike essential amino acids, non-essential amino acids can be tolerated in vivo, and mice fed diets lacking serine and glycine showed reduced tumor growth in p53-deficient cells. Serine starvation activates the SSP, leading to increased p53-independent expression of PHGDH and PSAT1. p53-deficient cells fail to proliferate under serine starvation, suggesting a defect in SSP enzyme expression. However, p53 may support SSP by down-regulating PGAM, allowing glycolytic intermediates to be channeled to the SSP. Both p53-proficient and -deficient cells can synthesize de novo serine, but p53-deficient cells have lower serine levels, indicating a defect in adaptation. Serine starvation reduces glycolysis, leading to lower ATP levels. Adding exogenous pyruvate partially restores proliferation in p53-deficient cells. Serine starvation also reduces GMP and AMP levels, leading to p53 activation and p21 expression. p53-deficient cells show reduced G1 arrest and impaired recovery of the cell cycle. p21 is critical for adaptation to serine starvation, and its loss exacerbates sensitivity to serine depletion. Serine starvation affects the balance between purine-nucleotide and glutathione synthesis. p53-proficient cells show increased flux to GSH synthesis, whileSerine starvation induces stress and p53-dependent metabolic remodeling in cancer cells. Cancer cells rely on specific metabolic adaptations to survive stress and support proliferation. The tumor suppressor p53 plays a key role in regulating various metabolic processes, including glycolysis, oxidative phosphorylation, glutaminolysis, and antioxidant responses. While p53 promotes apoptosis during DNA damage, it also supports cell survival during metabolic stress, which may contribute to both tumor suppression and non-cancer functions. This study shows that cancer cells rapidly utilize exogenous serine, and serine deprivation activates the serine synthesis pathway (SSP), reducing aerobic glycolysis and increasing flux to the TCA cycle. Transient p53-p21 activation and cell cycle arrest promote survival by channeling depleted serine to glutathione synthesis, maintaining antioxidant capacity. p53-deficient cells fail to respond to serine depletion, leading to oxidative stress, reduced viability, and impaired proliferation. In vivo models confirm that serine depletion can target p53-deficient tumors. Serine and glycine are essential nutrients for cancer cell proliferation, and their removal significantly reduces cell viability. p53-deficient cells show greater sensitivity to serine and glycine depletion than p53-proficient cells. Analysis of cell culture media shows rapid serine consumption by both p53-proficient and -deficient cells, while glycine uptake is low. Cancer cells avidly consume serine, which can be converted into methyl-THF and glycine via high SHMT expression. Unlike essential amino acids, non-essential amino acids can be tolerated in vivo, and mice fed diets lacking serine and glycine showed reduced tumor growth in p53-deficient cells. Serine starvation activates the SSP, leading to increased p53-independent expression of PHGDH and PSAT1. p53-deficient cells fail to proliferate under serine starvation, suggesting a defect in SSP enzyme expression. However, p53 may support SSP by down-regulating PGAM, allowing glycolytic intermediates to be channeled to the SSP. Both p53-proficient and -deficient cells can synthesize de novo serine, but p53-deficient cells have lower serine levels, indicating a defect in adaptation. Serine starvation reduces glycolysis, leading to lower ATP levels. Adding exogenous pyruvate partially restores proliferation in p53-deficient cells. Serine starvation also reduces GMP and AMP levels, leading to p53 activation and p21 expression. p53-deficient cells show reduced G1 arrest and impaired recovery of the cell cycle. p21 is critical for adaptation to serine starvation, and its loss exacerbates sensitivity to serine depletion. Serine starvation affects the balance between purine-nucleotide and glutathione synthesis. p53-proficient cells show increased flux to GSH synthesis, while