Inclusion Criteria:
1. Participants with post HMA-failure high-risk MDS (Int-2 or high risk by the IPSS with overall score \>/=1.5) with excess blasts \>5% with failure defined as prior receipt of 4 cycles of HMA therapy with failure to attain a response, or progression of disease or relapse at any time after prior response to HMA therapy.
2. Participants with relapsed/refractory chronic myelomonocytic leukemia (CMML) and therapy-related MDS are also eligible.
3. Hydroxyurea is allowed to lower the white cell count \</= 10,000/µl prior to initiation of venetoclax.
4. Adequate hepatic function including total bilirubin ≤ 2.0 x upper limit of normal (ULN) unless increase is due to Gilbert's disease or leukemic involvement, and ALT/AST ≤ 3.0 x ULN unless considered due to leukemic involvement.
5. Adequate renal function as calculated using the modified Cockcroft-Gault equation of ≥ 30ml/min, OR creatinine \< 2x ULN, unless related to the disease.
6. Signed written informed consent.
7. Females must be surgically or biologically sterile or postmenopausal (amenorrheic for at least 12 months) or if of childbearing potential, must have a negative serum or urine pregnancy test within 72 hours before the start of the treatment. Women of childbearing potential must agree to use an adequate method of contraception during the study and until 3 months after the last treatment.
8. Males must be surgically or biologically sterile or agree to use an adequate method of contraception during the study until 3 months after the last treatment.
9. Age \>/= 18 years of age.
10. ECOG/PS ≤2
Exclusion Criteria:
1. Participants having received any prior BCL2 inhibitor therapy.
2. Participants with MDS with IPSS risk categories Low or Int-1 (overall IPSS score \< 1.5).
3. Participants with known HIV infection (due to potential drug-drug interactions between antiretroviral medications and venetoclax). HIV testing will be performed at screening, only if required per local guidelines or institutional standards.
4. Participants known to be positive for hepatitis B or C infection \[HCV Ab indicative of a previous or current infection; and/or positive HBs Ag or detected sensitivity on HBV-DNA PCR test for HBc Ab and/or HBs Ab positivity\] with the exception of those with an undetectable viral load within 3 months of screening. (Hepatitis B or C testing is not required). Subjects with serologic evidence of prior vaccination to HBV \[i.e., HBs Ag-, and anti-HBs+\] may participate.
5. Participants has received strong and/or moderate CYP3A inducers within 7 days prior to the initiation of study treatment.
6. Participants has consumed grapefruit, grapefruit products, Seville oranges (including marmalade containing Seville oranges) or Starfruit within 3 days prior to the initiation of study treatment.
7. Participants has a cardiovascular disability status of New York Heart Association Class \> 2. Class 2 is defined as cardiac disease in which patients are comfortable at rest but ordinary physical activity results in fatigue, palpitations, dyspnea, or anginal pain.
8. Participant has chronic respiratory disease that requires continuous oxygen, or significant history of renal, neurologic, psychiatric, endocrinologic, metabolic, immunologic, hepatic, cardiovascular disease, any other medical condition or known hypersensitivity to any of the study medications including excipients of azacitidine that in the opinion of the investigator would adversely affect his/her participating in this study.
9. Participant has a malabsorption syndrome or other condition that precludes enteral route of administration.
10. Participant exhibits evidence of other clinically significant uncontrolled systemic infection requiring therapy (viral, bacterial or fungal).
11. Participant has received a live attenuated vaccine within 4 weeks prior to the first dose of study drug.
12. Participant has a history of other malignancies within 2 years prior to study entry, with the exception of:
* Adequately treated in situ carcinoma of the cervix uteri or carcinoma in situ of breast;
* Basal cell carcinoma of the skin or localized squamous cell carcinoma of the skin;
* Previous malignancy confined and surgically resected (or treated with other modalities) with curative intent; requires discussion with TA MD.
13. Participant has a white blood cell count \> 10 × 109/L. (Hydroxyurea or leukapheresis are permitted to meet this criterion)
14. Female subject has positive results for pregnancy test.
15. Participants with (Grade \> 1) unresolved from prior treatment (including chemotherapy, targeted therapy, immunotherapy, experimental agents, radiation, or surgery)
