The specific BCL-2 small molecule inhibitor venetoclax has been shown to effectively induce apoptosis in a wide range of malignancies either alone and in combination with other drugs. Based upon these results, there has been considerable growth in its use in clinical studies used alone and in combination with chemotherapy and immune-based therapies. Lympho- cytes, and T cells in particular, rely heavily on BCL-2 for survival and function. This has been determined largely by genetic deletion or overexpression of BCL-2 in murine models. However, the adaptive effects of short or long-term small molecule BCL-2 family blockade on surviving immune cell subsets and their function is not fully understood. In the current work, we aimed to better understand the effect of long-term systemic treatment with venetoclax on regulatory T (Treg) cells, which are relatively apoptotically resistant to specific BCL-2 drugging compared to other T cells. Our results indicate that long-term BCL-2 blockade by venetoclax results in Treg plasticity towards a TH17-like functional state which aids in anti-PD-1 efficacy. Our data further highlights the importance of Treg instability on the efficacy of anti-PD-1 checkpoint blockade which will be beneficial for the clinical efficacy of immunotherapy.