Treatment for the lethal primary adult brain tumor glioblastoma (GBM) includes the chemotherapy temozolomide (TMZ), but TMZ resistance is common and correlates with promoter methylation of the DNA repair enzyme O-6-methylguanine-DNA methyltransferase (MGMT). To improve treatment of GBMs, including those resistant to TMZ, we explored the potential of targeting dopamine receptor signaling. We found that dopamine receptor 3 (DRD3) is expressed in GBM and is also a previously unexplored target for therapy. We identified novel antagonists of DRD3 that decreased the growth of GBM xenograft-derived neurosphere cultures with minimal toxicity against human astrocytes and/or induced pluripotent stem cell-derived neurons. Among a set of DRD3 antagonists, we identified two compounds, SRI-21979 and SRI-30052, that were brain penetrant and displayed a favorable therapeutic window analysis of The Cancer Genome Atlas data demonstrated that higher levels of DRD3 (but not DRD2 or DRD4) were associated with worse prognosis in primary, MGMT unmethylated tumors. These data suggested that DRD3 antagonists may remain efficacious in TMZ-resistant GBMs. Indeed, SRI-21979, but not haloperidol, significantly reduced the growth of TMZ-resistant GBM cells. Together our data suggest that DRD3 antagonist-based therapies may provide a novel therapeutic option for the treatment of GBM.