The breast and ovarian tumor suppressor BRCA1 forms a heterodimeric RING-type ubiquitin ligase with BARD1 to catalyze untraditional Lys-6-linked polyubiquitin chains. It is not clear how the BRCA1-BARD1 ligase regulates various cellular processes such as DNA repair, cell-cycle progression, transcriptional regulation, and centrosome duplication. Here we report that BRCA1-BARD1 catalyzes the polyubiquitination of nucleolar phosphoprotein nucleophosmin/B23 (NPM). Two different mass spectrometry screens for protein ubiquitinated by BRCA1-BARD1 both identified NPM. NPM interacts with N-terminal fragments of BRCA1 and BARD1 in a manner dependent upon BRCA1-BARD1 heterodimer formation. NPM colocalizes with BRCA1 and BARD1 in mitotic cells suggesting the possibility of NPM regulation by BRCA1-BARD1 during mitosis. BRCA1-BARD1 catalyzes the ubiquitination of NPM in vitro and in vivo, and BRCA1-BARD1 co-expression in cells causes NPM stabilization rather than degradation. This is consistent with the notion that this ligase catalyzes untraditional polyubiquitin chains. Given the many overlapped functions between NPM and BRCA1, we propose that NPM is a strong candidate as a substrate of the BRCA1-BARD1 ubiquitin ligase.