Inclusion Criteria:  Male or female subjects ≥18 years of age at Screening. Subject must be able to provide informed consent. Subject has a confirmed diagnosis of primary ITP according to the International Consensus Report on the Investigation and Management of Primary ITP within the previous 6 months prior to Visit 1 and has had a previous response to a first line treatment (corticosteroids, IVIg, or anti-D), in the opinion of the Investigator. Subject has at least one platelet count <30×10⁹/L at any time during the screening period or at the Baseline visit. Females of childbearing potential must have a negative pregnancy test at Screening and Baseline. Female subjects of childbearing potential who are sexually active and male subjects who are sexually active must agree to use effective methods of contraception. Subject is willing and able to comply with all aspects of the protocol.  Exclusion Criteria:  Thrombocytopenia due to a known condition other than primary ITP (e.g., systemic lupus erythematosus, H. pylori infection, splenomegaly, chronic liver disease). Any history of arterial or venous thrombosis, including partial or complete thrombosis (history of superficial thrombophlebitis is not exclusionary). Subjects with known inherited thrombocytopenia (e.g., MYH-9 disorders). History of myelodysplastic syndrome (MDS) or other hematologic malignancies. Current history of significant cardiac arrhythmias or decompensated congestive heart failure. History of hepatitis B (HBV), hepatitis C (HCV), or human immunodeficiency virus (HIV). Previous use of eltrombopag, romiplostim, recombinant human TPO or other platelet-producing agents. Surgical resection of the spleen. Previous use of mycophenolate mofetil (MMF), rituximab (or other B-cell lymphocyte depleting agents), mercaptopurine (6-MP) or alkylating agents. Concurrent malignant disease, other than non-melanoma skin cancer or cervical cancer in-situ. Known allergy to avatrombopag or any of its excipients. Subject is unable to take oral medication or has a malabsorption syndrome or any other uncontrolled gastrointestinal condition. Enrollment in another clinical study with any investigational drug or device within 30 days of Day 1/Visit 2 (or 5 half-lives, whichever is longer); however, participation in observational studies is permitted. Any clinically relevant abnormality which makes the subject unsuitable for participation in the study, in the opinion of the Investigator. Considered unable or unwilling to comply with the study protocol requirements.