design: To systematically investigate the genomic features associated with benefit from anti-PD(L)1 in isocitrate dehydrogenase (IDH)-wildtype glioblastoma (GBM), we profiled 181 GBM cases (43 newly diagnosed, 138 recurrent) by whole exome sequencing (WES), targeted panel sequencing, bulk RNA-sequencing (RNA-Seq), and single-nucleus RNA sequenincg (snRNA-Seq). In multivariate survival analysis, baseline tumor transcriptional subtype and presence of their associated genetic lesions emerged as key features predicting overall survival (OS) among newly-diagnosed GBM treated with anti-PD(L)1, with the mesenchymal (MES) subtype associated with improved OS and the proneural subtype associated with poorer OS. Exposure to immune checkpoint blockade (ICB) was associated with outgrowth of subclones harboring lesions associated with non-MES subtype (CDKN2A deletions, PDGFRA gain-of-function mutations), which was not observed in analogous analyses of an external ICB-naive cohort. Our studies support the notion of an MES to non-MES transition as a common trajectory following ICB treatment for GBM. This study systematically characterized the genomic and transcriptomic features of isocitrate dehydrogenase (IDH)-wildtype glioblastoma (GBM) in the context of immune checkpoint blockade (ICB) therapy. A cohort of 181 ICB-treated GBM cases, encompassing both newly diagnosed and recurrent disease, was characterized using a combination of whole exome sequencing (WES), targeted panel sequencing, bulk RNA sequencing (RNA-seq), and single-nucleus RNA sequencing (snRNA-seq). WES was performed on matched tumor-normal tissue using Twist and ICE exome capture. Bulk RNA-seq was performed on tumor samples using Illumina TruSeq Stranded mRNA or TruSeq RNA Access protocols. snRNA-seq was performed on nuclei isolated from snap-frozen tumor specimens using the 10x Genomics Chromium platform. Together, these data were used to identify molecular determinants of ICB response and resistance, including associations between tumor transcriptional subtypes, immune cell composition, HLA expression, and clinical outcomes. Specimens collected across multiple treatment timepoints permitted investigation of clonal and transcriptional changes associated with ICB exposure. Note: The sequencing data for this sample corresponds to the snRNA-seq dataset.