•Cryo-EM structure of human BARD1 in complex with H2AK15-ubiquitinated nucleosome•The BUDR motif contacts the nucleosome acidic patch and ubiquitin K63-E64•The ARD domain grips H4 tail by accommodating unmethylated H4K20•Mutations disrupting BARD1-NCPUb association impair homologous recombination The BRCA1-BARD1 directs DNA double-strand break (DSB) repair pathway choice to error-free (HR) duri...