Updated project metadata. DNA replication stress is the major cause of genomic instability in human cells. The ataxia telangiectasia and Rad3-related kinase (ATR) plays an essential role in the cellular response to replication stress and inhibition of ATR has emerged as therapeutic strategy for the treatment of cancer. However, the ATR-dependent signaling in the cellular response to replication stress has not been systematically investigated. Here, we employ quantitative mass spectrometry-based proteomics to decipher the ATR-dependent phosphorylation events in response to pathological replication stress induced by hydroxyurea. Our data define the substrate spectrum of ATR and identify several novel putative ATR substrates. In addition, we demonstrate that ATR-inhibition fundamentally rewires cellular signaling networks leading to the prominent activation of different pathways including the ATM-driven double strand break repair.