Nutrient availability is a key determinant of tumor cell behavior. While nutrient‐rich conditions favor proliferation and tumor growth, scarcity, and particularly glutamine starvation, promotes cell dedifferentiation and chemoresistance. Here, linking ribosome biogenesis plasticity with tumor cell fate, we uncover that the amino acid sensor general control non‐derepressible 2 (GCN2; also known as eIF‐2‐alpha kinase 4) represses the expression of the precursor of ribosomal RNA (rRNA), 47S, under metabolic stress. We show that blockade of GCN2 triggers cell death by an irremediable nucleolar stress and subsequent TP53‐mediated apoptosis in patient‐derived models of colon adenocarcinoma (COAD). In nutrient‐rich conditions, a cell‐autonomous GCN2 activity supports cell proliferation by stimulating 47S rRNA transcription, independently of the canonical integrated stress response (ISR) axis. Impairment of GCN2 activity prevents nuclear translocation of methionyl‐tRNA synthetase (MetRS), resulting in nucleolar stress, mTORC1 inhibition and, ultimately, autophagy induction. Inhibition of the GCN2–MetRS axis drastically improves the cytotoxicity of RNA polymerase I (RNA pol I) inhibitors, including the first‐line chemotherapy oxaliplatin, on patient‐derived COAD tumoroids. Our data thus reveal that GCN2 differentially controls ribosome biogenesis according to the nutritional context. Furthermore, pharmacological co‐inhibition of the two GCN2 branches and RNA pol I activity may represent a valuable strategy for elimination of proliferative and metabolically stressed COAD cells.