• Male or female Chinese participants aged 12 or older at the time of informed consent. Written informed consent from participant, parent or legal guardian.

• Able to swallow tablets.

• Have undergone alloSCT from any donor source (matched unrelated donor, sibling, haplo-identical) using bone marrow, peripheral blood stem cells, or cord blood.

• Clinically diagnosed Grades II to IV acute GvHD as per standard criteria occurring after alloSCT requiring systemic immune suppressive therapy.

• Evident myeloid and platelet engraftment (confirmed within 48 hours prior to study treatment (ruxolitinib) start):

• Confirmed diagnosis of steroid refractory aGvHD defined as participants administered high-dose systemic corticosteroids (methylprednisolone 2 mg/kg/day \[or equivalent prednisone dose 2.5 mg/kg/day\]), given alone or combined with calcineurin inhibitors (CNI) and either:

‣ Progression based on organ assessment after at least 3 days compared to organ stage at the time of initiation of high-dose systemic corticosteroid +/- CNI for the treatment of Grade II to IV aGvHD. OR

⁃ Failure to achieve at a minimum partial response based on organ assessment after 7 days compared to organ stage at the time of initiation of high-dose systemic corticosteroid +/-CNI for the treatment of Grade II to IV. OR

⁃ Participants who fail corticosteroid taper defined as fulfilling either one of the following criteria:

• Requirement for an increase in the corticosteroid dose to methylprednisolone ≥2 mg/kg/day (or equivalent prednisone dose ≥2.5 mg/kg/day). OR

∙ Failure to taper the methylprednisolone dose to \< 0.5 mg/kg/day (or equivalent prednisone dose \<0.6 mg/kg/day) for a minimum of 7 days.