Background: Environmental stress has an important role in the course of bipolar disorder. Some findings have shown that family beliefs about the illness could predict family burden, and this burden could influence the outcome of bipolar disorder. To the best of our knowledge, there is scant information about the effects of family intervention on the caregiver’s burden in bipolar disorder. The aim of this study was to assess the effects of psychoeducational family intervention on bipolar patients’ caregivers, including the assessment of the caregiver’s burden. Methods: 45 medicated euthymic bipolar outpatients were randomized into an experimental and a control group. Relatives of patients from the experimental group received 12 psychoeducational, 90-min sessions about bipolar disorder and coping skills. The caregivers’ knowledge of bipolar disorder, the relationship subscales of the Family Environment Scale, and the family burden subscales from an adapted version of the Social Behavior Assessment Schedule were assessed for both caregiver groups before and after the intervention. Results: Psychoeducated caregivers significantly improved their knowledge of bipolar disorder and reduced both the subjective burden and the caregiver’s belief about the link between the objective burden and the patient. No significant differences were found in the objective burden nor in the family relationship subscales. Conclusions: These preliminary results suggest that psychoeducational intervention on caregivers of bipolar patients may improve the caregiver’s knowledge of the illness, reduce their distress or subjective burden and alter their beliefs about the link between the disruptions in their life and the patient’s illness.