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Cancer cells display high levels of DNA damage and replication stress, vulnerabilities that could be exploited by drugs targeting DNA repair proteins. Human CtIP promotes homology-mediated repair of DNA double-strand breaks (DSBs) and protects stalled replication forks from nucleolytic degradation, thus representing an attractive candidate for targeted cancer therapy. Here, we establish a peptide mimetic of the CtIP tetramerization motif that inhibits CtIP activity. The hydrocarbon-stapled peptide encompassing amino acid residues 18 to 28 of CtIP (SP$^{18-28}$) stably binds to CtIP tetramers in vitro and facilitates their aggregation into higher-order structures. Efficient intracellular uptake of SP$^{18-28}$ abrogates CtIP localization to damaged chromatin, impairs DSB repair, and triggers extensive fork degradation. Moreover, prolonged SP$^{18-28}$ treatment causes hypersensitivity to DNA-damaging agents and selectively reduces the viability of BRCA1-mutated cancer cell lines. Together, our data provide a basis for the future development of CtIP-targeting compounds with the potential to treat patients with cancer.