Background: Injecting drug users are at high risk of HIV infection globally. Research related to female drug users is rare in Kenya, yet it is required to inform the development of gender-sensitive HIV prevention and harm reduction services in East Africa, where injecting drug use is on the rise. Methods: This study aimed to document the nature of HIV risks encountered by women who inject drugs in the Mombasa and Kilifi, Kenya. Secondary data analysis was conducted on an existing dataset from a 2015 primary qualitative study involving 24 interviews and 3 focus group discussions with 45 women who inject drugs. These were complemented with five interviews with key stakeholders involved in the provision of services to women who inject drugs. Guided by the social ecology theory, a thematic analysis was conducted to identify the nature of HIV risks and their underlying determinants. Results: HIV risk behaviours fell into two broad categories: unsafe injecting and unprotected sex. These risks occurred in the form of sharing of needles, unprotected oral, anal and vaginal sex, sexual assaults, injecting drug use during sex, sex work, and other types of transactional sex. The primary determinants underlying these risks were a low-risk perception, inequitable gender power, economic pressures, and poor availability of needles and condoms. These social-ecological these determinants did not exist in isolation, but intersected with each other to create powerful influences which exposed women to HIV. Social-ecological determinants exerted constant influence and created a persistent ‘HIV risk environment’ that was involuntarily experienced by women. Conclusion: Individual, interpersonal, and societal-structural factors intersect to produce HIV risk behaviours. As a minimum, these risks will require a combination of multifaceted micro-level interventions including self-efficacy training, risk assessment skills, couple counselling, and universal access to the recommended harm reduction package. In addition, the current focus on micro-level interventions in Kenya needs to shift to incorporate macrolevel interventions, including livelihood, employability, and gender norms-transforming interventions, to mitigate economic and gender-related drivers of HIV risks. In the Kenyan context, injecting drug use during sex work is emerging as an increasingly important HIV risk behaviour needing to be addressed.