Background Successful parenting requires maternal behaviors that promote infant survival such as protection from predators. In animal studies, oxytocin (OT) has been linked to maternal aggression to protect offspring. No human study has explored this topic. Mothers with a diagnosis of postnatal depression (PND) are at higher risk of neglecting their infants. We hypothesized that intranasal OT administration would increase the protective behaviors of mothers with PND, toward their infants. Methods Sixteen mothers with a diagnosis of PND participated in a double‐blind, randomized‐controlled, within‐subject pilot study. Participants received intranasal OT during one visit and placebo spray on the alternate visit. Maternal protective behavior toward their infant was measured, in the presence of a socially intrusive stranger. Results The enthusiastic stranger paradigm stimulated participants' protective responses in the presence of an intrusive stranger. Furthermore, this protective response of mothers with a diagnosis of PND was increased in the OT condition. Conclusions The study introduces a new paradigm, the enthusiastic stranger paradigm, which may be used to examine a neglected type of parental behavior, that is, protection of offspring. The protective response of mothers with PND increased, in line with the ‘tend and defend’ effects of OT in animal models. In future work it should be tested whether this protection effect can also be found in nonclinical samples, or whether it is specific for clinically depressed mothers.