We introduce the concept of “menstrual poverty” to categorize the multiple deprivations relating to menstruation in resource-poor settings across the Global South, and we examine how this affects the psychological well-being of adolescent girls in an urban informal settlement in Kenya. We use qualitative data collected through 34 in-depth interviews and 18 focus group discussions with girls, women, and key informants. Menstrual poverty involved practical and psychosocial challenges affecting girls at home and at school. Its emotional impacts included anxiety, embarrassment, fear of stigma, and low mood. Further research is needed on how menstrual poverty affects girls’ psychological and educational outcomes.