Abstract Background Aggressive behavior is a challenging behavior among bipolar patients that causes poor social interaction and hospitalization. But, there is no information regards of the magnitude and contributing factors for aggressive behaviour among bipolar patients in Ethiopia. Therefore, this study was designed to assess the prevalence and associated factors of aggressive behaviour among patients with bipolar disorder. Method An institutional based cross sectional study was conducted at Amanual Mental Specialized Hospital from May 1 to June 1, 2015 among 411 participants who were selected by systematic random sampling technique. Data was collected by interview technique by using Modified Overt Aggression Scale, entered and analyzed by using Epi Data 3.1 and Statistical Package for Social Science version 20, respectively. Adjusted Odd Ratio (AOR) with 95% Confidence Interval (CI) were used to show the odd and P -value