Activating mutations in NOTCH1 are common in T cell acute lymphoblastic leukemia (T-ALL). Here we identify glutaminolysis as a critical pathway for leukemia cell growth downstream of NOTCH1 and a key determinant of the response to anti-NOTCH1 therapies in vivo. Mechanistically, inhibition of NOTCH1 signaling in T-ALL induces a metabolic shutdown, with prominent inhibition of glutaminolysis and triggers autophagy as a salvage pathway supporting leukemia cell metabolism. Consequently, inhibition of glutaminolysis and inhibition of autophagy strongly and synergistically enhance the antileukemic effects of anti-NOTCH1 therapy in mice harboring T-ALL. Moreover, we demonstrate that Pten loss upregulates glycolysis and consequently rescues leukemic cell metabolism, thereby abrogating the antileukemic effects of NOTCH1 inhibition. Overall, these results identify glutaminolysis as a major node in cancer metabolism controlled by NOTCH1 and as therapeutic target for the treatment of T-ALL.