Inclusion Criteria:

          -  Participants with documented diagnosis of intermediate-2 or high-risk primary
             Myelofibrosis, Post Polycythemia Vera Myelofibrosis or Post-essential Thrombocythemia
             Myelofibrosis.

          -  Participant must be ineligible due to age, comorbidities, or unfit for unrelated or
             unmatched donor transplantation or unwilling to undergo stem cell transplantation at
             time of study entry.

          -  Eastern Cooperative Oncology Group (ECOG) of 0, 1, or 2.

          -  Prior treatment must meet at least one of the following criteria:

               -  Prior or current treatment with ruxolitinib and no prior treatment with a
                  Bromodomain and Extra-Terminal motif (BET) proteins inhibitor or another Janus
                  Kinase 2 (JAK-2) inhibitor, and meet all of the following criteria:

                    -  Ruxolitinib treatment must meet at least one of the following criteria:

                         -  Ruxolitinib treatment for >=24 weeks with lack of efficacy defined as a
                            lack of spleen response (refractory) or a loss of spleen or symptom
                            response (relapsed)

                         -  Ruxolitinib treatment for <24 weeks with documented disease progression
                            on spleen measurements while on ruxolitinib as defined in the protocol:

                         -  Ruxolitinib treatment for >=28 days with intolerance defined as new red
                            blood cell transfusion requirement (at least 2 units/month for 2
                            months) while receiving a total daily ruxolitinib dose of >=30 mg but
                            unable to reduce dose further due to lack of efficacy.

                    -  If receiving ruxolitinib at the time of screening, must currently be on a
                       stable dose >=10 mg twice daily of ruxolitinib for >=4 weeks prior to the
                       1st dose of navitoclax.

                    -  Participant has at least 2 symptoms each with a score >=3 or a total score
                       of >=12, as measured by the Myelofibrosis Symptom Assessment Form (MFSAF)
                       v4.0 on at least 4 out of 7 days during screening prior to study drug
                       dosing; OR

               -  Prior treatment with a JAK-2 inhibitor and meet one of the following criteria:

                    -  Prior treatment with a JAK-2 inhibitor for at least 12 weeks

                    -  Prior treatment with a JAK-2 inhibitor for >=28 days complicated by either
                       development of red blood cell transfusion requirement (at least 2
                       units/month for 2 months) OR Grade >= 3 adverse events of thrombocytopenia,
                       anemia, hematoma and/or hemorrhage while on JAK-2 inhibitor treatment; OR

               -  No prior treatment with a JAK-2 or BET inhibitor:

                    -  Participant has at least 2 symptoms each with a score >=3 or a total score
                       of >= 12, as measured by the MFSAF v4.0 on at least 4 out of 7 days during
                       screening prior to study drug dosing.

          -  Participant has splenomegaly as defined in the protocol.

          -  Participant must meet the laboratory parameters (adequate bone marrow, renal and
             hepatic function) as defined in the protocol.

        Exclusion Criteria:

          -  Splenic irradiation within 6 months prior to screening, or prior splenectomy.

          -  Leukemic transformation (> 10% blasts in peripheral blood or bone marrow
             aspirate/biopsy).

          -  Participant is currently on medications that interfere with coagulation (including
             warfarin) or platelet function within 3 days prior to the first dose of study drug or
             during the study treatment period with the exception of low dose aspirin (up to 100
             mg/day) and low-molecular-weight heparin.

          -  Prior therapy with a BH3 mimetic compound or stem cell transplantation.