Although poly(ADP-ribose) polymerase (PARP) inhibitors are active in homologous recombination (HR)-deficient cancers, their utility is limited by acquired resistance after restoration of HR. Here, we report that dinaciclib, an inhibitor of cyclin-dependent kinases (CDKs) 1, 2, 5, and 9, additionally has potent activity against CDK12, a transcriptional regulator of HR. In BRCA-mutated triple-negative breast cancer (TNBC) cells and patient-derived xenografts (PDXs), dinaciclib ablates restored HR and reverses PARP inhibitor resistance. Additionally, we show that de novo resistance to PARP inhibition in BRCA1-mutated cell lines and a PDX derived from a PARP-inhibitor-naive BRCA1 carrier is mediated by residual HR and is reversed by CDK12 inhibition. Finally, dinaciclib augments the degree of response in a PARP-inhibitor-sensitive model, converting tumor growth inhibition to durable regression. These results highlight the significance of HR disruption as a therapeutic strategy and support the broad use of combined CDK12 and PARP inhibition in TNBC. Display omitted •Dinaciclib is a potent inhibitor of CDK12 and disrupts homologous recombination•Residual HR is a cause of de novo PARP inhibitor resistance in BRCA-mutated cancer•Dinaciclib sensitizes resistant BRCA-mutated breast cancer models to PARP inhibition•In HR-deficient cancer, dinaciclib augments the response afforded by PARP inhibition Johnson et al. show that CDK12 inhibition, mediated by the small molecule dinaciclib, disrupts residual or restored homologous recombination (HR) in BRCA-associated breast cancer, sensitizing to PARP inhibition. CDK12 inhibition also augments the degree of PARP inhibitor response in HR-deficient breast cancer. Combined CDK12-PARP inhibition is well tolerated in preclinical models.