Introduction: Girls in Tanzania often experience menstrual restrictions (i.e., expectations about behaviors prohibited during menstruation) and menstrual stigma (i.e., negative attitudes toward people who menstruate). A better understanding of sociocultural contexts contributing to menstrual stigma and restrictions is needed. Methods: In 2018, two conceptually linked quantitative and qualitative studies were conducted to describe menstrual restrictions and stigma among girls in Tanzania, examine how sociocultural contexts influence menstrual restrictions and stigma, and investigate the relationship between menstrual restrictions and stigma. A cross-sectional survey of 509 post-menarche girls from five schools assessed associations between religion, religion-based menstrual restrictions, and menstrual stigma while controlling for differences between schools. In-depth interviews with 10 adolescent girls and 10 adult key informants explored broader menstrual experiences. We analyzed themes pertaining to sociocultural contexts and other mechanisms that shape menstrual restrictions and stigma, their impacts on girls, and opportunities for intervention. Results: In the surveys, 52% of Christian girls and 76% of Muslim girls experienced religion-based menstrual restrictions. Some restrictions, such as prohibitions against praying during menstruation, were more common among Muslim girls, who also had higher menstrual stigma. Menstrual stigma differed by school. Although menstrual restrictions did not show an association with stigma in the surveys, interview participants described how menstrual restrictions contribute to stigma and indicated that religion, tribe, education, family support, and menstrual resources influence menstrual restrictions and stigma. Conclusion: Sociocultural contexts influence menstrual restrictions and stigma. Working with religious, school, and family leaders may provide opportunities to implement interventions to reduce menstrual stigma.