Inclusion Criteria:
* Male and female participants 2 to \< 18 years of age as of Day 1.
* eGFR ≥ 30 mL/min/1.73m2 where eGFR is calculated using the modified Schwartz formula at Screening and confirmed during the Run-in Period.
* Kidney biopsy-proven primary IgAN\*, with biopsy performed within 3 years of Screening with \< 50% tubulointerstitial fibrosis and \< 25% crescents. In case a kidney biopsy within 3 years from Screening is not available, a kidney biopsy may be performed if it is part of the planned diagnostic approach and clinical management of the participant.
\* Note: Primary IgAN is defined as any IgAN that is proven by biopsy showing IgA deposits prevalent over the other classes of immunoglobulins and deemed not to be associated with causes of secondary IgAN as per clinical judgment of the Investigator.
* The minimum body weight for participants in Cohort 1 is 35 kg at Screening and confirmed at Baseline (Day 1).
* Proteinuria due to primary diagnosis of IgAN as assessed by UPCR ≥ 1 g/g (113 mg/mmoL) sampled from FMV at Screening on Day -90 and Day -60 as well as during the Run-in Period despite treatment with maximum tolerated dose of ACE inhibitor/ARB for at least 120 days prior to Day 1. Note: UPCR will be assessed based on one FMV sample at Day -90 and based on the geometric mean of 2 FMV samples for the Day -60 visit and during the Run-in Period.
* Vaccination against Neisseria meningitidis and Streptococcus pneumoniae infection is required prior to the start of study treatment. If the participant has not been previously vaccinated, or if a booster is required, vaccine should be given according to local guidelines at least 2 weeks prior to first study drug administration. If study treatment has to start earlier than 2 weeks post vaccination, prophylactic antibiotic treatment should be initiated.
* Vaccination against Haemophilus influenzae is recommended, according to local guidelines, at least 2 weeks before iptacopan.
* All participants must have been on supportive care including stable dose regimen of ACE inhibitor or ARB at either the locally approved maximal daily dose per body weight, or the maximally tolerated dose (per Investigator's judgment for pediatric use), for at least 120 days before first study drug administration. In addition, if participants are taking diuretics, other antihypertensive medication, or other background medication for IgAN (such as SGLT2 inhibitors), the doses should also be stabilized for at least 120 days prior to the first dosing of study treatment.
Exclusion Criteria:
* Any secondary IgAN observed at Screening (and confirmed at Baseline/Day 1) as defined by the Investigator; secondary IgAN can be associated with cirrhosis, celiac disease, human immunodeficiency virus (HIV) infection, herpes simplex virus infection, dermatitis herpetiformis, seronegative arthritis, small-cell carcinoma, lymphoma, disseminated tuberculosis, bronchiolitis obliterans, inflammatory bowel disease, and familial mediterranean fever.
* A clinical diagnosis of immunoglobulin A vasculitis (IgAV or Henoch-Schonlein purpura) based on typical palpable purpura with or without arthralgia and abdominal pain.
* Evidence of significant urinary obstruction or difficulty in voiding at Screening (and confirmed at Baseline/Day 1); any urinary tract disorder or any chronic kidney disease other than IgAN at Screening and before first study drug administration.
* Current acute kidney injury (AKI) defined by Acute Kidney Injury Network (AKIN) criteria within 4 weeks of screening.
* Presence of rapidly progressive glomerulonephritis (RPGN) as defined by 50% decline in eGFR within 3 months prior to Screening or during the Screening and Run-in periods.
* Presence of nephrotic syndrome at Screening based on the investigator's judgment.
* History or current diagnosis of ECG abnormalities indicating significant risk for study participants such clinically significant cardiac arrhythmias, e.g. sustained ventricular tachycardia, or clinically significant second- or third-degree atrioventricular (AV) block without a pacemaker.
* History or current diagnosis of clinically significant echocardiogram abnormalities indicating significant risk for study participants including but not limited to clinically significant functional, morphological and/or structural abnormalities, which may include left ventricular ejection fraction \< 50% and/or left ventricular hypertrophy due to uncontrolled blood pressure at Screening.
* Participants of 12 to \< 18 years of age with systolic blood pressure (SBP) \< 80 mmHg or \> 150 mmHg, or diastolic blood pressure (DBP) \< 50 mmHg or \> 95 mmHg, or pulse rate \< 50 bpm or \> 110 bpm; participants of 6 to \< 12 years of age with SBP \< 70 mmHg or \> 140 mmHg, or DBP \< 40 mmHg or \> 90 mmHg or pulse rate \< 52 bpm or \> 156 bpm; participants of 2 to \< 6 years of age with SBP \< 70 mmHg or \> 120 mmHg, or DBP \< 40 mmHg or \> 80 mmHg or pulse rate \< 52 bpm or \> 156 bpm at Screening.
* Participants previously treated with immunosuppressive or other immunomodulatory agents such as but not limited to cyclophosphamide, rituximab, infliximab, canakinumab, mycophenolate mofetil (MMF) or mycophenolate sodium (MPS), calcineurin inhibitors, complement inhibitors (other than iptacopan), oral budesonide, systemic corticosteroids exposure (≥ 0.5 mg/kg/day of prednisone/prednisolone equivalent or \> 7.5 mg/d prednisone/prednisolone equivalent total exposure in a single day) within 120 days (or 180 days for rituximab) prior to first study drug administration. Participants treated with endothelin (receptor) antagonists (including sparsentan) within 120 days prior to first study drug administration.
* All transplanted participants (any solid organ transplantation, including bone marrow transplantation).
* History of recurrent invasive infections caused by encapsulated organisms, such as meningococcus and pneumococcus.
* Major concurrent comorbidities at Screening including but not limited to advanced cardiac disease (e.g. New York Heart Association (NYHA) class III (for 6 to \<18 years of age), Ross class III (for 2 to \< 6 years of age), severe pulmonary disease (e.g. World Health Organization (WHO) class III (for 17 years of age); Pulmonary Vascular Research Institute (PVRI) class III (for 2 to \< 17 years of age), or hepatic disease (e.g. active hepatitis) that in the opinion of the investigator precludes participation in the study.
* Current use of any homeopathic and/or herbal medications for IgAN disease progression, such as but not limited to Lei Gong Teng at Screening (and confirmed at Baseline/Day 1).
Other protocol-defined inclusion/exclusion criteria may apply
