Street-based sexual harassment is a prevalent but understudied form of gender-based violence that restricts women’s access to public spaces. Drawing on adaptations of the ecological model that identify the root causes of gender-based violence in patriarchy, in this study, we explore the causes of street harassment in informal areas of Greater Cairo. Our analysis is based on qualitative interviews and focus groups with male and female youth aged 13–29 years, parents of youth and community leaders in two informal areas. We supplement the qualitative data with descriptive analysis of a representative, 2016 survey of youth in informal areas of Cairo that measured experiences with and attitudes toward harassment. Harassment was prevalent in the study areas, and respondents tended to place the blame for harassment at the individual level of the ecological model, particularly women’s behavior. However, there were also community- and societal-level factors that contributed to the prevalence of harassment. Patriarchal norms and stigmatization of women who are harassed reinforced victim-blaming, such that most young women were afraid to report experiences of harassment due to social censure. In this context, educational or awareness raising interventions are unlikely to be effective in combating harassment, which is widely acknowledged to be a problem. Rather, interventions are needed along the different levels of the ecological model to target peer group and community norms that encourage harassment, address harassment in schools and strengthen reporting mechanisms.