Accurate chromosomal segregation is monitored by the mitotic checkpoint, and an increased rate of chromosomal missegregation leads to chromosomal instability (CIN). Here, we demonstrate that the HBV X protein (HBx) binds BubR1, a component of the mitotic checkpoint complex and co-localizes with BubR1 at the kinetochores. HBx binding to BubR1 attenuates the association between BubR1 and CDC20, an activator of the anaphase-promoting complex/cyclosome (APC/C) and induces slippage of mitotic arrest in the presence of microtubule poisons. In addition, HBx binding to BubR1 results in the accumulation of lagging chromosomes and chromosome bridges. In contrast, a C-terminally truncated HBx mutant (HBx(1-100)) fails to bind BubR1 and does not cause aberrant chromosomal segregation. This provides a novel mechanism for dysregulation of the mitotic checkpoint by a viral pathogen linking it to the accumulation of chromosomal instability in HBV-associated hepatocarcinogenesis.