Background: Prior studies have suggested that inhibited temperament may be associated with an increased risk for developing anxiety or mood disorder, including bipolar disorder. However, the neurobiological basis for this increased risk is unknown. The aim of this study was to examine temperament in symptomatic and asymptomatic child offspring of parents with bipolar disorder (OBD) and to investigate whether inhibited temperament is associated with aberrant hippocampal volumes compared with healthy control (HC) youth. Methods: The OBD group consisted of 45 youth, 24 of whom had current psychiatric symptoms (OBD(+)s) and 21 without any psychiatric symptoms (OBD-s), and were compared with 24 HC youth. Temperament characteristics were measured by using the Revised Dimensions of Temperament Survey. Magnetic resonance imaging was used to measure hippocampal volumes. The association between temperament and hippocampal volumes was tested by using multiple regression analysis. Results: Compared with the OBD-s group, the OBD(+)s group had significantly more inhibited temperament traits, less flexibility, more negative mood, and less regular rhythm in their daily routines. In contrast, the OBD(-)s group was more likely to approach novel situations compared with OBD(+)s or HC groups. Within the OBD(+)s group, a more inhibited temperament was associated with smaller right hippocampal volumes. Conclusions: In this study, symptomatic OBD were characterized by an inhibited temperament that was inversely correlated with hippocampal volume. Additional longitudinal studies are needed to determine whether inverse correlations between hippocampal volume and inhibited temperament represent early markers of risk for later developing bipolar disorder.