This is a global, multicenter, 2-part study to evaluate the efficacy and safety of selinexor plus ruxolitinib in JAK inhibitor (JAKi) treatment-naïve myelofibrosis (MF) participants. The study will be conducted in two phases: Phase 1 (open-label) and Phase 3 (double-blind). Phase 1 (enrollment completed) was an open-label evaluation of the safety and recommended Phase 2 dose (RP2D) of selinexor in combination with ruxolitinib and included a dose escalation using a standard 3+3 design (Phase 1a) and a dose expansion part (Phase 1b). Phase 3 (ongoing), double-blind, placebo-controlled part of the study comparing the efficacy and safety of combination therapy of selinexor + ruxolitinib with combination of placebo + ruxolitinib.

Inclusion Criteria:

• Aged ≥ 18 years
• A diagnosis of primary MF or post-essential thrombocythemia (ET) or postpolycythemia- vera (PV) MF.
• Active symptoms of MF as determined by presence of at least 2 symptoms using the Myelofibrosis Symptom Assessment Form (MFSAF) V4.0.
• Participants with international prognostic scoring system (DIPSS) risk category of intermediate-1, or intermediate-2, or high-risk.
• Measurable splenomegaly during the screening period as demonstrated by spleen volume of greater than or equal to (>=) 450 cubic centimeter (cm^3) .
• Eastern Cooperative Oncology Group (ECOG) Performance Status less than or equal to (<=) 2.

Exclusion Criteria:

• More than 10% blasts in peripheral blood or bone marrow (accelerated or blast phase).
• Previous treatment with JAK inhibitors for MF.
• Previous treatment with selinexor or other XPO1 inhibitors.