Research investigating online (hetero)sexual harassment among young people often reports on male perpetrators and female victims, with limited inquiry into female perpetrators of such violence. In order to develop broader understandings of online (hetero)sexual harassment, it is necessary to move beyond the gender binary of male perpetrators and female victims. This chapter examines the complex ways in which black girls perpetrate online (hetero)sexual harassment. Thirty boys and girls at a South African high school, aged 13–18, participated in semi-structured individual interviews and focus group discussions. The findings demonstrate how girls’ sexual agency is influenced by unequal gendered systems within a heterosexual matrix. Girls strategically use this matrix to harass those whom they construct as not adhering to normative femininity and masculinity. Using a new feminist materialist approach, we explore the interconnectedness of real and online worlds. Girls indirectly harass other girls anonymously and use fake profiles, while boys are directly harassed by girls with real profiles who strategically seek to emasculate boys alongside specific conventions of hegemonic masculinity. This study reveals how online (hetero)sexual harassment cohering around a heterosexual matrix ultimately constrains young people’s healthy sexual becomings, thus necessitating gender-transformative interventions to address sexual harassment among young people in online spaces.