The histone lysine demethylase KDM5B regulates gene transcription and cell differentiation. It contains three PHD fingers, the biological roles of which remain elusive. Here, we show that the first PHD1 finger of KDM5B binds unmodified histone H3, whereas the third PHD3 finger prefers the trimethylated mark, H3K4me3. RNA-seq analysis indicates that KDM5B functions as a transcriptional repressor for a set of genes. Biochemical analysis reveals that KDM5B associates with components of the nucleosome remodeling and deacetylase (NuRD) complex and may cooperate with HDAC1 in gene repression. Compared with the estrogen receptor positive breast cancers, KDM5B is downregulated in the triple-negative breast cancer. Overexpression of KDM5B in the MDA-MB 231 breast cancer cells suppresses cell migration and invasion ability, and the PHD1-H3K4me0 interaction is important for inhibition of migration. These findings highlight tumor-suppressive functions of KDM5B in triple-negative breast cancer cells and suggest a novel multivalent mechanism for KDM5B-mediated transcriptional regulation.