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The BCL2 inhibitor venetoclax has been approved to treat different hematological malignancies. Since there is no common genetic alteration causing resistance to venetoclax in CLL and B cell lymphoma, we asked if epigenetic events might be involved in venetoclax resistance. Therefore, we employed whole exome sequencing, methylated DNA immunoprecipitation sequencing and genome wide CRISPR/Cas9 screening to investigate venetoclax resistance in aggressive lymphoma and high-risk CLL patients. We identified a regulatory CpG island within the PUMApromoter which is methylated upon venetoclax treatment, mediating PUMAdownregulation on transcript and protein level. PUMAexpression and sensitivity towards venetoclax can be restored by inhibition of methyltransferases. We can demonstrate that loss of PUMA results in metabolic reprogramming with higher OXPHOS and ATP production, resembling the metabolic phenotype that is seen upon venetoclax resistance. While PUMA loss is specific for acquired venetoclax resistance but not for acquired MCL1 resistance and is not seen in CLL patients after chemotherapy-resistance, BAX is essential for sensitivity towards both venetoclax and MCL1 inhibition. As we found loss of BAX in Richter’s syndrome patients after venetoclax failure, we defined BAX-mediated apoptosis to be critical for drug resistance but not for disease progression of CLL into aggressive DLBCL in vivo. A compound screen revealed TRAIL-mediated apoptosis as a target to overcome BAX deficiency. Furthermore, antibody or CAR T cells eliminated venetoclax resistant lymphoma cells, paving a clinically applicable way to overcome venetoclax resistance.