The purpose of this study to confirm the selexipag starting dose(s), selected based on pharmacokinetic (PK) extrapolation from adults, that leads to similar exposure as adults doses in children from greater than or equal to (>=) 2 to less than (˂) 18 years of age with Pulmonary Arterial Hypertension (PAH), by investigating the PK of selexipag and its active metabolite ACT-333679 in this population.

The selection of the starting dose for pediatric participants is based on the PK extrapolation from adults, taking into account the children body weight category, in order to lead to an exposure similar to that in adult PAH participants at a starting dose of 200 micrograms (mcg). As in adults, selexipag will be up-titrated to the individual maximum tolerated dose (iMTD) during the first 12 weeks. Approximately 60 participants will be enrolled in 3 different age cohorts to obtain at least 45 participants with evaluable PK profiles: Cohort 1: >= 12 to < 18 years of age, Cohort 2: >= 6 to < 12 years of age, Cohort 3: >= 2 to < 6 years of age. In each age cohort the starting dose will depend on the body weight. Enrollment will start with both Cohort 1 and Cohort 2. After completion of PK assessments in at least 15 participants from Cohort 1 at Week 12, a first interim analysis will be conducted to establish the dose-exposure relationship using a population PK model. The PK data from any participants in Cohort 2 who have completed their PK assessments at this time will be included in this first interim analysis. Results of this model-based analysis will be used to confirm or adjust the selexipag doses initially selected. Enrollment of Cohort 3 (children >= 2 to < 6 years of age) will start once the appropriate doses have been confirmed in a second interim analysis of PK data from Cohorts 1 and 2, and if there is no safety concern based on review by an Independent Data Monitoring Committee (IDMC).

Inclusion Criteria:

• Signed and dated informed consent by the parent(s) or Legally authorized representative(s) AND assent from developmentally capable children

• Males or females between greater than or equal to (>=) 2 and less than (<) 18 years of age with weight >= 9 kilograms (kg)

• Pulmonary arterial hypertension (PAH) diagnosis confirmed by documented historical right heart catheterization (RHC) performed at any time before participant's enrollment

• PAH with one of the following etiologies:

  • idiopathic (iPAH),
  • heritable (hPAH),
  • associated with congenital heart disease (CHD): PAH with co-incidental CHD; post-operative PAH (persisting/ recurring/ developing >= 6 months after repair of CHD)
  • Drug or toxin-induced
  • PAH associated with HIV
  • PAH associated with connective tissue disease

• Word Health Organization functional class (WHO FC) II to III

• Participants treated with an endothelin receptor antagonist (ERA) and/or a phosphodiesterase type 5 (PDE-5) inhibitor provided that the treatment dose(s) has been stable for at least 3 months prior to enrollment, or participants who are not candidates for these therapies

• Females of childbearing potential must have a negative pregnancy test at Screening and at Enrollment, and must agree to undertake monthly pregnancy tests, and to use a reliable method of contraception (if sexually active) from screening up to study drug discontinuation plus 30 days (EOS)

Key Exclusion Criteria:

• Participants with PAH due to portal hypertension, schistosomiasis, pulmonary veno-occlusive disease (PVOD) and/or pulmonary capillary hemangiomatosis
• Participants with PAH associated with Eisenmenger syndrome
• Participants with moderate to large left-to-right shunts
• Participants with cyanotic congenital cardiac lesions such as transposition of the great arteries, truncus arteriosus, univentricular heart or pulmonary atresia with ventricular septal defect, as well as Participants with Fontan-palliation
• Participants with pulmonary hypertension due to lung disease
• Previous treatment with Uptravi (selexipag) within 2 weeks prior to enrollment
• Participants having received prostacyclin (epoprostenol) or prostacyclin analogs (that is, treprostinil, iloprost, beraprost) within 2 months prior to enrollment or are scheduled to receive any of these compounds during the trial
• Treatment with another investigational drug within 4 weeks prior to enrollment
• History, or current suspicion of intussusception or ileus or gastrointestinal obstruction as per investigator's judgment
• Uncontrolled thyroid disease as per investigator judgment
• Hemoglobin or hematocrit < 75 percentage (%) of the lower limit of normal range
• Known severe or moderate hepatic impairment
• Clinical signs of hypotension that in the investigator's judgment would preclude initiation of a PAH-specific therapy
• Participants with severe renal insufficiency
• Known hypersensitivity to the investigational treatment or to any of the excipients of the drug formulations