The purpose of this study is to assess the efficacy and safety of the study drug, levosimendan (given orally), compared to placebo in participants with pulmonary hypertension with heart failure with preserved left ventricular ejection fraction (PH-HFpEF) as measured by the change in 6-Minute Walk Distance.

Approximately 540 participants will be randomized in a 2:1 ratio to receive an oral dose of levosimendan or placebo. Participants will be eligible for an open label extension (OLE) of 52-weeks.

Inclusion Criteria:

1. Men or women, ≥18 to 85 years of age
2. NYHA Class II or III or ambulatory NYHA Class IV symptoms
3. A diagnosis of World Health Organization (WHO) Group 2 PH-HFpEF with qualifying hemodynamics verified by right heart catheterization (RHC)
4. A qualifying baseline RHC
5. A qualifying echocardiogram
6. A qualifying 6-MWD
7. A 48-hour ambulatory cardiac rhythm monitor during the Screening Period
8. Requirements related to child bearing potential, contraception, and egg/sperm donation)

Exclusion Criteria:

1. A diagnosis of PH WHO Groups 1, 3, 4, or 5
2. Echocardiographic evidence for hypertrophic cardiomyopathy, restrictive cardiomyopathy, constrictive pericarditis, cardiac amyloidosis, or infiltrative cardiomyopathy
3. Structural heart repair or replacement of the aortic valve or mitral valve (surgical or percutaneous) OR, planned valve intervention OR, the presence of significant valve disease
4. A diagnosis of pre-existing lung disease
5. History of severe allergic or anaphylactic reaction or hypersensitivity to the excipients in the investigational product
6. Major surgery within 60 days
7. Prior heart, lung, or heart-lung transplants or life expectancy of <12 months
8. History of clinically significant other diseases that may limit or complicate participation in the study