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.

Study of Selinexor in Combination With Ruxolitinib in Myelofibrosis