Inclusion Criteria:

• Signed and dated informed consent form.
• Male and female subjects from 18 to 85 years old (inclusive).
• 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.
• Subject 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.

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 randomization.
• Treatment with prostacyclin (epoprostenol), prostacyclin analogs (i.e., treprostinil, iloprost, beraprost) or prostacyclin receptor agonists (i.e., selexipag) within 90 days prior to randomization.
• Change in dose or initiation of new diuretics and/or calcium channel blockers within 1 week prior to baseline RHC.
• 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).