Immune checkpoint therapy (ICT) using antibody blockade of programmed cell death protein 1 (PD-1) or cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) can provoke T cell–dependent antitumor activity that generates durable clinical responses in some patients. The epigenetic and transcriptional features that T cells require for efficacious ICT remain to be fully elucidated. Herein, we report that anti–PD-1 and anti–CTLA-4 ICT induce upregulation of the transcription factor BHLHE40 in tumor antigen–specific CD8⁺ and CD4⁺ T cells and that T cells require BHLHE40 for effective ICT in mice bearing immune-edited tumors. Single-cell RNA sequencing of intratumoral immune cells in BHLHE40-deficient mice revealed differential ICT-induced immune cell remodeling. The BHLHE40-dependent gene expression changes indicated dysregulated metabolism, NF-κB signaling, and IFNγ response within certain subpopulations of CD4⁺ and CD8⁺ T cells. Intratumoral CD4⁺ and CD8⁺ T cells from BHLHE40-deficient mice exhibited higher expression of the inhibitory receptor gene Tigit and displayed alterations in expression of genes encoding chemokines/chemokine receptors and granzyme family members. Mice lacking BHLHE40 had reduced ICT-driven IFNγ production by CD4⁺ and CD8⁺ T cells and defects in ICT-induced remodeling of macrophages from a CX3CR1⁺CD206⁺ subpopulation to an iNOS⁺ subpopulation that is typically observed during effective ICT. Although both anti–PD-1 and anti–CTLA-4 ICT in BHLHE40-deficient mice led to the same outcome—tumor outgrowth—several BHLHE40-dependent alterations were specific to the ICT that was used. Our results reveal a crucial role for BHLHE40 in effective ICT and suggest that BHLHE40 may be a predictive or prognostic biomarker for ICT efficacy and a potential therapeutic target.