This study will evaluate the efficacy, safety, pharmacokinetics, and pharmacodynamics of ravulizumab in adult and adolescent participants with hematopoietic stem cell transplant-associated thrombotic microangiopathy (HSCT-TMA). In Stage 1, an open-label, single-arm period, the dosing regimen will be confirmed. In Stage 2, participants will be randomized to receive either blinded ravulizumab plus best supportive care or matching placebo plus best supportive care. The treatment period is 26 weeks (open-label for Stage 1, and randomized, double-blind, and placebo-controlled for Stage 2) followed by a 26-week follow-up period.

Inclusion Criteria:

1. 12 years of age or older at time of consent/assent.
2. Received HSCT within the past 12 months.
3. Diagnosis of TMA that persists for at least 72 hours after initial management of any triggering agent/condition.
4. A TMA diagnosis based on meeting the laboratory-based criteria during the Screening Period and/or ≤14 days prior to the Screening Period.
5. Body weight ≥ 30 kilograms at Screening or ≤7 days prior to the start of the Screening Period (date of consent).
6. Female participants of childbearing potential and male participants with female partners of childbearing potential must use highly effective contraception.
7. Participants must be vaccinated against meningococcal infections if clinically feasible. Participants who cannot receive meningococcal vaccine should receive antibiotic prophylaxis. Participants <18 years of age must be re-vaccinated against Haemophilus influenzae type b (Hib) and Streptococcus pneumoniae if clinically feasible.
8. Participants or their legally authorized representative must be capable of giving signed informed consent or assent.

Exclusion Criteria:

1. Thrombotic thrombocytopenic purpura (TTP) evidenced by ADAMTS13 deficiency
2. Known Shiga toxin-related hemolytic uremic syndrome as demonstrated by positive test.
3. Positive direct Coombs test indicative of a clinically significant immune-mediated hemolysis not due to TMA.
4. Clinical diagnosis of disseminated intravascular coagulation (DIC).
5. Known bone marrow/graft failure for the current HSCT.
6. Diagnosis of veno-occlusive disease which is unresolved at the time of Screening.
7. Human immunodeficiency virus (HIV) infection.
8. Unresolved meningococcal disease.
9. Presence of sepsis requiring vasopressor support.
10. Pregnancy or breastfeeding.
11. Hypersensitivity to murine proteins or to one of the excipients of ravulizumab.
12. Any ongoing or history of medical or psychological conditions unrelated to HSCT-TMA that could increase the risk to the participant or confound the outcome of the study.
13. Respiratory failure requiring mechanical ventilation.
14. Acute and/or chronic heart failure with an ejection fraction ≤ 40%.
15. Previously or currently treated with a complement inhibitor.
16. Participation in an interventional treatment study of any therapy for TMA.