• Diagnosis of primary myelofibrosis (PMF) according to the revised World Health Organization (WHO) criteria or post-essential thrombocythemia-MF or post-polycythemia vera according to the International Working Group for Myelofibrosis Research and Treatment (IWG-MRT) criteria.

• Dynamic International Prognostic Scoring System (DIPSS) intermediate-1, intermediate-2 or high-risk MF.

• Candidate for ruxolitinib treatment:

‣ Part 1 participants: On ruxolitinib treatment for at least 12 weeks with at least 4 consecutive weeks immediately prior to enrollment at a stable dose.

⁃ Part 2 participants: Candidate for ruxolitinib treatment as assessed by the investigator and has not previously been treated with a JAK inhibitor (Cohort A) OR currently receiving JAK inhibitor treatment per standard of care for at least 12 weeks (maximum 48 weeks) with at least 4 consecutive weeks at a stable dose prior to enrollment (Cohort B).

• Clinical signs/symptoms of MF demonstrated by one of the following:

‣ Measurable splenomegaly demonstrated by either a palpable spleen measuring ≥5 cm below the left costal margin or a spleen volume ≥450 cm\^3 by MRI or CT, AND

⁃ active symptoms of MF on the MFSAF v4.0.

• Ineligible for or unwilling to undergo hematopoietic stem cell transplant at time of study entry.

• Hematology laboratory test values within protocol defined limits.

• Biochemical laboratory test values within protocol defined limits.

• Eastern Cooperative Oncology Group Performance Status score of 0, 1, or 2.

• Participants should follow protocol defined contraceptives procedures.

• A woman of childbearing potential must have a negative serum or urine pregnancy test at screening.