Social reward is critical for social relationships, and yet we know little about the characteristics of social interactions that are rewarding or the neural mechanisms underlying that reward. Furthermore, sex differences in the neural mechanisms mediating social reward likely contribute to the sex differences in the prevalence and predisposition to many psychiatric disorders. Here, using a variety of behavioral, pharmacological, neuroendocrine and molecular approaches we investigate the behavioral characteristics underlying the rewarding properties of same-sex social interactions and the sex-dependent role of the oxytocin system in regulating the magnitude and valence of social reward. We found 1) that there may be an inverted U shaped dose response relationship between the duration of social interaction and social reward value, 2) females find same-sex social interactions more rewarding than males and 3) the OT system is necessary for social reward, in both males and females and depending on the social context “social dose”, activation of OTRs in the VTA can increase social reward in males, but have the opposite effect, decrease social reward in females. Collectively, these studies provide support for the hypothesis there is an inverted U relationship between the duration of social interaction and social reward, and that females may be more sensitive to the rewarding effects of social interactions. Furthermore, the OT system mediates social reward in males and females, and more specifically, OT can have the opposite effect on social reward in males and females. In conclusion, understanding these sex differences in social reward processing may be essential for understanding the sex differences in the prevalence of many psychiatric disorders and the development of sex-specific treatments of neuropsychiatric disorders.