Chronic social defeat affects many aspects of behavior. Most previous studies have focused on effects on males and defeat during adolescence. The extents to which chronic social defeat can impact female social behavior in adulthood and the neural mechanisms of such effects are poorly understood. Using highly social and aggressive female mandarin voles (Microtus mandarinus), the present study found that chronic social defeat reduced social preference in adult females, and that the defeated voles exhibited a high level of freeze, self-grooming and defensive behavior, as well as reduced exploration, intimacy and aggression during social interactions. Furthermore, chronic social defeat reduced levels of oxytocin (OT) and OT receptors (OTR) in the shell region of the nucleus accumbens (NACC). Intra-NACC shell OT microinjections reversed alterations in social behavior induced by chronic social defeat, whereas injections of an OTR antagonist (OTR-A) blocked the effects of OT. Taken together, our data demonstrate that chronic social defeat suppresses measures of sociability, and that these effects are mediated by the action of OT on the OTR in the NACC. NACC OT may be a promising target to treat socio-emotional disorders induced by chronic social stress.