Indoleamine 2,3-dioxygenase 1 (IDO1) is an immunosuppressive protein that inhibits antitumor immunity through both tryptophan metabolism and nonenzymatic functions. Drugs targeting IDO1 enzyme activity have failed to improve the overall survival of patients with cancer. Developing new therapeutics that neutralize both enzyme- and nonenzyme-derived immunosuppressive IDO1 effects is therefore of high interest. We previously described a novel proteolysis targeting chimera (PROTAC), NU223612, that degrades IDO1 in cultured human glioblastoma (GBM) cells, as well as in well-established brain tumors, in vivo. In this study, we rationally optimized the structure of our lead series to create NU227326, which degrades IDO1 with a DC50 of 5 nM in human GBM cells. Mechanistic studies showed that IDO1 degradation occurred through the ubiquitin–proteasome system and was sustained for at least 2 days, supporting NU227326 as a highly potent IDO1 PROTAC suitable for further studies in GBM and other human cancers.