The WEE1 kinase negatively regulates CDK1/2 to control DNA replication and mitotic entry. Genetic factors that determine sensitivity to WEE1 inhibitors (WEE1i) are largely unknown. A genome-wide insertional mutagenesis screen revealed that mutation of EIF2A, a translation regulator, sensitized to WEE1i. Additionally, a genome-wide CRISPR-Cas9 screen revealed that inactivation of integrated stress response (ISR) kinase GCN2 or its co-factor GCN1 rescued WEE1i-mediated cytotoxicity. Conversely, loss of the collided ribosome sensor ZNF598 increased sensitivity to WEE1i. Mechanistically, WEE1i induced paradoxical GCN2 activation, ATF4 upregulation, and altered ribosome dynamics. ISR activation was independent of WEE1 presence, pointing at off-target GCN2 engagement by multiple chemically distinct WEE1i. ISR activation was observed in cancer cells as well as non-transformed cells, and required GCN1 and ongoing translation. Consequently, WEE1i induce multiple independent cellular effects: DNA damage, premature mitotic entry and sensitization to DNA-damaging chemotherapeutics in an ISR-independent fashion, as well as ISR activation independently of CDK1/2 activation. Importantly, low-dose WEE1 inhibition did not induce ISR activation, while it still synergized with PKMYT1 inhibition. Taken together, WEE1i trigger toxic ISR activation and translational shutdown, which can be prevented by low-dose or combination treatments, while retaining the cell cycle checkpoint-perturbing effects.