Isocitrate dehydrogenase ( )-mutant glioma is a distinct glioma molecular subtype for which no effective molecularly directed therapy exists. Low-grade gliomas, which are 80%-90% -mutant, have high RNA levels of the cell surface Notch ligand DLL3. We sought to determine DLL3 expression by IHC in glioma molecular subtypes and the potential efficacy of an anti-DLL3 antibody-drug conjugate (ADC), rovalpituzumab tesirine (Rova-T), in -mutant glioma. We evaluated expression by RNA using TCGA data and by IHC in a discovery set of 63 gliomas and 20 nontumor brain tissues and a validation set of 62 known wild-type and mutant gliomas using a monoclonal anti-DLL3 antibody. Genotype was determined using a DNA methylation array classifier or by sequencing. The effect of Rova-T on patient-derived endogenous -mutant glioma tumorspheres was determined by cell viability assay. Compared to wild-type glioblastoma, -mutant gliomas have significantly higher RNA ( < 1 × 10 ) and protein by IHC ( = 0.0014 and < 4.3 × 10 in the discovery and validation set, respectively). DLL3 immunostaining was intense and homogeneous in -mutant gliomas, retained in all recurrent tumors, and detected in only 1 of 20 nontumor brains. Patient-derived -mutant glioma tumorspheres overexpressed DLL3 and were potently sensitive to Rova-T in an antigen-dependent manner. DLL3 is selectively and homogeneously expressed in -mutant gliomas and can be targeted with Rova-T in patient-derived -mutant glioma tumorspheres. Our findings are potentially immediately translatable and have implications for therapeutic strategies that exploit cell surface tumor-associated antigens.