Study ROR-PH-303 is a multicenter, open-label extension (OLE) study for subjects with WHO Group 1 PAH who have participated in another Phase 2 or Phase 3 study of ralinepag and who qualify for entry based upon participation in the prior study. Subjects who discontinue participation in a prior study due to safety issues related to study medication or who fail to complete study procedures will not be eligible to enter Study ROR-PH-303. Subjects who meet all OLE entry criteria will be enrolled and will receive ralinepag in addition to their pre-existing PAH disease-specific background therapy. For subjects who are enrolled from an ongoing double-blind Phase 2 or Phase 3 ralinepag study, a 16-week blinded Dose Titration Period will be completed following enrollment into the OLE. Subjects previously on ralinepag will continue on the same dose during the Dose Titration Period as received in the original study and will also initiate placebo treatment in the OLE. Subjects who are enrolled from a non-blinded study or another OLE will not participate in the blinded Dose Titration Period in Study ROR-PH-303 but will be enrolled directly into the Treatment Period and continue on the dose of ralinepag received in the original study. All subjects will receive ralinepag in the OLE study until premature discontinuation of ralinepag due to an AE/serious adverse event (SAE) or other reason, marketing approval of ralinepag is granted in the region in which the study is conducted, or the study is discontinued by the Sponsor.