The histone lysine demethylase KDM5B regulates gene transcription and cell differentiation and is implicated in carcinogenesis. It contains multiple conserved chromatin-associated domains, including three PHD fingers of unknown function. Here, we show that the first and third, but not the second, PHD fingers of KDM5B possess histone binding activities. The PHD1 finger is highly specific for unmodified histone H3 (H3K4me0), whereas the PHD3 finger shows preference for the trimethylated histone mark H3K4me3. RNA-seq analysis indicates that KDM5B functions as a transcriptional repressor for genes involved in inflammatory responses, cell proliferation, adhesion, and migration. Biochemical analysis reveals that KDM5B associates with components of the nucleosome remodeling and deacetylase (NuRD) complex and may cooperate with the histone deacetylase 1 (HDAC1) in gene repression. KDM5B is downregulated in triple-negative breast cancer relative to estrogen-receptor-positive breast cancer. Overexpression of KDM5B in the MDA-MB 231 breast cancer cells suppresses cell migration and invasion, and the PHD1-H3K4me0 interaction is essential for inhibiting migration. These findings highlight tumor-suppressive functions of KDM5B in triple-negative breast cancer cells and suggest a multivalent mechanism for KDM5B-mediated transcriptional regulation. Display omitted •KDM5B binds to its histone target and enzymatic product through distinct PHD fingers•KDM5B may cooperate with the NuRD complex in transcriptional repression•KDM5B suppresses migration and invasion of triple-negative breast cancer cells The histone lysine demethylase KDM5B regulates gene transcription and cell differentiation. Here, Shi, Kutateladze, and colleagues find that the first and third PHD fingers of KDM5B have histone-binding activities. The authors show that KDM5B is downregulated in triple-negative breast cancer cells, functions as a transcriptional repressor for a set of genes, and suppresses cell migration and invasion ability. These findings highlight tumor-suppressive functions of KDM5B in triple-negative breast cancer cells and suggest a multivalent mechanism for KDM5B-mediated transcriptional repression.