Background: Women in mathematical domains may become attuned to situational cues that signal a discredited social identity, contributing to their lower achievement and underrepresentation. Aim: The current study examined whether heightened in-group representation alleviates the effects of stereotype threat on women’s mathematical performance. It further investigated whether single-sex testing environments and stereotype threat influenced participants to believe that their ability was fixed (fixed mindset) rather than a trait that could be developed (growth mindset). Sample and Method: One hundred and forty-four female participants were assigned randomly to a self-as-target or group-as-target stereotype threat condition or to a control condition. They completed a modular arithmetic maths test and a mindset questionnaire either alone or in same-sex groups of 3-5 individuals. Results: Participants solved fewer mathematical problems under self-as-target and group-as-target stereotype threat when they were tested alone but these performance deficits were eliminated when they were tested in single-sex groups. Participants reported a weaker growth mindset when they were tested under stereotype threat and in single-sex groups. Moreover, evidence of inconsistent mediation indicated that single-sex testing environments negatively predicted mindset but positively predicted mathematical performance. Conclusions: These findings suggest that single-sex testing environments may represent a practical intervention to alleviate stereotype threat effects but may have a paradoxical effect on mindset.