The study will be conducted in compliance with the International Council for Harmonisation (ICH) of Technical Requirements for Registration of Pharmaceuticals for Human Use/Good Clinical Practice (GCP) and applicable regulatory requirements. This is a Phase 1/2 multicenter, single arm, open-label study in Japanese subjects with DIPSS intermediate or high-risk PMF, post-PV or post-ET MF. The study consists of 2 parts: Phase 1 part to determine safety and tolerability and a RP2D. The Phase 1 portion of the study will explore one or more drug doses for fedratinib (300 mg and 400 mg) using a mTPI-2 design. Following completion of dose escalation and determination of MTD and/or a RP2D, the study will progress into the Phase 2 part to further evaluate the efficacy and safety. The study will consist of 3 periods: a Screening Period, a Treatment Period including a 30-day follow-up after last dose visit and a survival follow-up period.