Mutations in SF3B1 have been identified across several cancer types. This key spliceosome component promotes the efficient mRNA splicing of thousands of genes including those with crucial roles in the cellular response to DNA damage. Here, we demonstrate that depletion of SF3B1 specifically compromises homologous recombination (HR) and is epistatic with loss of BRCA1. More importantly, the most prevalent cancer-associated mutation in SF3B1, K700E, also affects HR efficiency and as a consequence, increases the cellular sensitivity to ionising radiation and a variety of chemotherapeutic agents, including PARP inhibitors. Additionally, the SF3B1 K700E mutation induced unscheduled R-loop formation, replication fork stalling, increased fork degradation and defective replication fork restart. Taken together, these data suggest that tumour-associated mutations in SF3B1 induce a BRCA-like cellular phenotype that confers synthetic lethality to DNA damaging agents and PARP inhibitors, which can be exploited therapeutically.