Ataxia telangiectasia (A‐T) is a human disease caused by ATM deficiency characterized among other symptoms by radiosensitivity, cancer, sterility, immunodeficiency and neurological defects. ATM controls several aspects of cell cycle and promotes repair of double strand breaks (DSBs). This probably accounts for most of A‐T clinical manifestations. However, an impaired response to reactive oxygen species (ROS) might also contribute to A‐T pathogenesis. Here, we show that ATM promotes an anti‐oxidant response by regulating the pentose phosphate pathway (PPP). ATM activation induces glucose‐6‐phosphate dehydrogenase (G6PD) activity, the limiting enzyme of the PPP responsible for the production of NADPH, an essential anti‐oxidant cofactor. ATM promotes Hsp27 phosphorylation and binding to G6PD, stimulating its activity. We also show that ATM‐dependent PPP stimulation increases nucleotide production and that G6PD‐deficient cells are impaired for DSB repair. These data suggest that ATM protects cells from ROS accumulation by stimulating NADPH production and promoting the synthesis of nucleotides required for the repair of DSBs.