Myc oncoproteins induce tumor initiation and promote tumor progression by modulating gene transcription. We have previously shown that N-Myc represses gene transcription by recruiting histone deacetylases to Sp1-binding site-enriched regions of target gene promoters. The histone demethylase JARID1B plays a dual role in cancer. In the present study, we examined published microarray gene expression datasets and found that JARID1B was commonly repressed by Myc oncoproteins and histone deacetylases in cancer cell lines of various organ origins. Chromatin immunoprecipitation assays demonstrated that N-Myc repressed JARID1B expression by direct binding to the Sp1-binding site-enriched region of the JARID1B gene promoter, and cell proliferation assays showed that transcriptional repression of JARID1B reduced neuroblastoma cell proliferation. Our findings suggest that Myc-mediated transcriptional repression of JARID1B counterintuitively inhibits Myc-regulated cell proliferation and potentially tumorigenesis.