A Study to Find Out if Selexipag is Effective and Safe in Patients With Chronic Thromboembolic Pulmonary Hypertension When the Disease is Inoperable or Persistent/Recurrent After Surgery and/or Interventional Treatment

Main Inclusion Criteria: * Signed and dated informed consent form * Male and female participants from greater than or equal to (\>) 18 (or the legal age of consent in the jurisdiction in which the study is taking place) and less then or equal to (\<=85) years old at Screening (Visit 1) * With established diagnosis of inoperable CTEPH (i.e., technically non-operable) or persistent/recurrent CTEPH after pulmonary endarterectomy (PEA) and/or balloon pulmonary angioplasty (BPA), as confirmed by the corresponding adjudication committee * With pulmonary hypertension (PH) in WHO FC I-IV. * Participant able to perform the 6-minute walk test (6MWT) with a minimum distance of 100 m and a maximum distance of 450 m at screening visit. * Women of childbearing potential must have a negative pregnancy test at screening and randomization and must agree to undertake monthly urine pregnancy tests, and to use a reliable method of birth control from screening visit up to at least 30 days after study treatment discontinuation. If a hormonal contraceptive is chosen it must be taken for at least 1 month prior to randomization. Main Exclusion Criteria: * Planned or current treatment with another investigational treatment up to 3 months prior to randomization. * Any known factor or disease that might interfere with treatment compliance, study conduct, or interpretation of the results, such as drug or alcohol dependence or psychiatric disease. * Known concomitant life-threatening disease with a life expectancy \< 12 months. * Planned balloon pulmonary angioplasty within 26 weeks after randomization. * Change in dose or initiation of new PH-specific therapy within 90 days prior to the baseline RHC (and LHC, if needed) qualifying for enrollment for the hemodynamic cohort and within 90 days prior to randomization (Visit 2) for the non-hemodynamic cohort * Treatment with prostacyclin (epoprostenol), prostacyclin analogs (i.e., treprostinil, iloprost, beraprost) or prostacyclin receptor agonists (i.e., selexipag) within 90 days prior to randomization (visit 2) except those given at vasodilator testing during RHC * Change in dose or initiation of new diuretics and/or calcium channel blockers within 1 week prior to baseline RHC (and LHC, if needed) * Any co-morbid condition that may influence the ability to perform a reliable and reproducible 6MWT, including use of walking aids (cane, walker, etc). * Any other criteria as per selexipag Summary of Product Characteristics (SmPC). * Exclusion criteria related to comorbidities: severe coronary heart disease or unstable angina as assessed by the investigator; mocardial infarction within the last 6 months prior to or during Screening; decompensated cardiac failure if not under close supervision; severe arrhythmias as assessed by the investigator; cerebrovascular events (example transient ischemic attack, stroke) within the last 3 months prior to or during screening; congenital or acquired valvular defects with clinically relevant myocardial function disorders not related to pulmonary hypertension. (PH); known or suspicion of pulmonary veno-occlusive disease