Inclusion Criteria:

          -  Diagnosis of primary myelofibrosis according to the revised World Health Organization
             criteria or post-essential thrombocythemia-MF or post-polycythemia vera-MF according
             to the IWG-MRT criteria

          -  Dynamic International Prognostic Scoring System intermediate-2 or high-risk MF

          -  Refractory to JAK-inhibitor treatment as defined in either inclusion (i) or (ii):

             (i) Treatment with JAK-inhibitor for >= 6 months duration, including at least 2 months
             at an optimal dose as assessed by the investigator for that participant and one of the
             following:

               1. no decrease in spleen volume (< 10% by MRI or CT) from the start of treatment
                  with JAK-inhibitor

               2. no decrease in spleen size (< 30% by palpation or length by imaging) from the
                  start of treatment with JAK-inhibitor

               3. no decrease in symptoms (< 20% by MFSAF or myeloproliferative neoplasm SAF) from
                  the start of treatment with JAK-inhibitor)

               4. a score of at least 15 on TSS assessed using the MFSAF v4.0 during screening.

                  (ii) Treatment with JAK-inhibitor treatment for >= 3 months duration with maximal
                  doses (e.g., 20-25 mg twice daily ruxolitinib) for that participant and no
                  decrease in spleen volume/size or symptoms as defined in inclusion criterion (i
                  [a, b, or c])

          -  Measurable splenomegaly demonstrated by a palpable spleen measuring >= 5 cm below the
             left costal margin or a spleen volume >= 450 cm^3 by MRI or CT

          -  Active symptoms of MF on the MFSAF v4.0 demonstrated by a symptom score of at least 5
             points (on a 0 to 10 scale)

          -  Hematology laboratory test values within the protocol defined limits

          -  Biochemical laboratory test values must be 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

        Exclusion Criteria:

          -  Peripheral blood blast count of >= 10% or bone marrow blast count of >=10%

          -  Known allergies, hypersensitivity, or intolerance to imetelstat or its excipients

          -  Prior treatment with imetelstat

          -  Any chemotherapy or MF directed therapy, including investigational drug regardless of
             class or mechanism of action, immunomodulatory or immunosuppressive therapy,
             corticosteroids greater than 30 mg/day prednisone or equivalent, and JAK-inhibitor
             treatment less than equal to 14 days prior to randomization

          -  Diagnosis or treatment for malignancy other than MF except:

               -  Malignancy treated with curative intent and with no known active disease present
                  for >= 3 years before randomization

               -  Adequately treated non-melanoma skin cancer or lentigo maligna without evidence
                  of disease

               -  Adequately treated cervical carcinoma in situ without evidence of disease

          -  Known history of human immunodeficiency virus or any uncontrolled active systemic
             infection requiring IV antibiotics

          -  Active systemic hepatitis infection requiring treatment (carriers of hepatitis virus
             are permitted to enter the study), or any known acute or chronic liver disease unless
             related to underlying hepatosplenomegaly due to MF

          -  Major surgery within 28 days prior to randomization

          -  Any life-threatening illness (e.g., coronavirus disease-2019), medical condition, or
             organ system dysfunction which, in the investigator's opinion, could compromise the
             participant's safety, interfere with the imetelstat metabolism, or put the study
             outcomes at undue risk