Ruxolitinib is a useful treatment option for myelofibrosis since it effectively resolves splenomegaly and constitutional symptoms. After the widespread use of ruxolitinib outside of clinical trials, a series of case reports indicated a potential risk of ruxolitinib-associated opportunistic infections, including the reactivation of the hepatitis B virus (HBV). We herein report the case of a polycythemia vera patient who showed an elevation of HBV-DNA viral DNA with an elevation of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) after the initiation of ruxolitinib. Anti-viral therapy with entecavir was immediately started and the HBV viral load thereafter decreased with an improvement of the liver function. Physicians should thus be aware of the potential risk of ruxolitinib-associated HBV reactivation.