Updated project metadata. Immunotherapies targeting cancer-specific neoantigens have revolutionized the treatment of cancer patients. Recent evidence suggests that epigenetic therapies synergize with immunotherapies, mediated bythe de-repression of endogenous retroviral element (ERV)-encoded promoters, and the initiation of transcription. Here, we use deep RNA sequencing from cancer cell lines treated with DNA methyltransferase inhibitors (DNMTi) and/or Histone deacetylase transferase inhibitors (HDACi), to assemble a de novo transcriptome and identified 3,023 ERV-derived, treatment-induced novel polyadenylated transcripts (TINPATs), encoding for 61,426 open reading frames. Using Immunopeptidomics, we further demonstrate the human leukocyte antigen (HLA) presentation of treatment-induced neoepitopes (t-neoepitopes) derived from TINPATs. We illustrate the potential of the identified t-neoepitopes to elicit a T-cell response and cancer cell killing. The presence of t-neoepitopes was further verified in AML patient samples 48/96 h after in vivo treatment with the DNMT inhibitor Decitabine. Our findings highlight a novel mechanism of ERV-derived neoantigens in epigenetic and immune therapies.