Inclusion Criteria:

1. Participants will be male and female with primary ITP with duration of >6 months in pediatric participants aged 12 to <18 years (pediatric participants aged 10 to <12 years will be enrolled in the EU [EEA countries] only) and duration of >3 months in ages 18 years and above
2. Participants who had a response (achievement of platelet count ≥50,000/µL) to IVIg/anti-D or CSs that was not sustained and who have documented intolerance, insufficient response or any contraindication to any appropriate courses of standard of care ITP therapy
3. An average of 2 platelet counts at least 5 days apart of <30,000/µL during the Screening period and no single platelet count >35,000/µL, within 14 days prior to the first dose of study drug.
   • Pediatric participants must additionally be determined to need treatment for ITP as per clinical assessment by the Investigator.
4. Adequate hematologic, hepatic, and renal function (absolute neutrophil count ≥1.5 ×

   10^9/L, AST/ALT ≤1.5 × upper limit of normal [ULN], albumin ≥3 g/dL, total bilirubin ≤1.5 × ULN [unless the partcipant has documented Gilbert syndrome], glomerular filtration rate >50 [Cockcroft and Gault method])

5. Hemoglobin >9 g/dL within 1 week prior to Study Day 1
6. All contraceptive use by men and women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies
7. Participants must be able to provide written informed consent or informed assent with corresponding informed consent obtained from the participants' guardian and agree to the schedule of assessments.

Exclusion Criteria:

1. Participants with secondary ITP
2. Pregnant or lactating women
3. History (within 5 years of Study Day 1) or current, active malignancy requiring or likely to require chemotherapeutic or surgical treatment during the study, with the exception of non melanoma skin cancer
4. Transfusion with blood, blood products, plasmapheresis, or use of any other rescue medications with intent to increase platelet count within 14 days before Study Day 1
5. Change in CS and/or TPO-RA dose within 14 days prior to Study Day 1 (more than 10% variation from current doses)
6. Immunosuppressant drugs other than CSs within 5 times the elimination half-life of the drug or 14 days of Study Day 1, whichever is longer
7. Treatment with rituximab or splenectomy within the 3 months prior to Study Day 1
   • Participants treated with rituximab will have normal B-cell counts prior to enrollment
8. Has received any investigational drug within the 30 days before receiving the first

   dose of study medication, or at least 5 times elimination half-life of the drug (whichever is longer); participant should not be using an investigational device at the time of dosing

   • Participants who previously received treatment with Bruton's Tyrosine Kinase (BTK) inhibitors (except rilzabrutinib) within 30 days before the first dose of study drug are not eligible
   • Participants who previously received rilzabrutinib at any time are not eligible
9. History of solid organ transplant
10. Myelodysplastic syndrome
11. Live vaccine within 28 days prior to Study Day 1 or plan to receive one during the study
12. Planned surgery in the time frame of the dosing period