Background The northeastern region of Nigeria has had a persistent increase in the number of internally displaced people due to insurgency. The disruption to the socioeconomic lives of the women in this region places them at a high risk of all forms of abuse from insurgents, security agents deployed to protect the people in the camps and host communities, and other members of the community. This study aims to assess the prevalence and pattern related to sexual and gender-based violence in selected states in northeast Nigeria. The study also assessed the availability and patronage of health-care services by survivors of sexual and gender-based violence and facilities for the prosecution of perpetrators of such violence. Methods A mixed methods study design was used. We obtained quantitative data using a semi-structured questionnaire and a facility checklist administered by interviewers to internally displaced people and workers at health facilities. Qualitative data were recorded in focus group discussions and in-depth interviews were used to collect data from internally displaced women, community leaders, security personnel, health workers in the camps and host communities, and staff of non-governmental and humanitarian organisations. Findings We included data from 4868 internally displaced people. About a third had experienced a form of sexual violence (1616 [33•2%]) while a fifth reported physical violence (997 [20•5%]). We noted that 1382 women (28•4%) had experienced socioeconomic violence and 1484 (30•5%) reported emotional violence, while about half reported harmful traditional practices (2297 [47•2%]). Experience of sexual and physical violence since displacement was reported by 123 (7•6%) and 36 women (3•7%), respectively. Perpetrators of sexual violence since displacement include Boko Haram insurgents (63 [51•5%]); 34 (27•3%) were unknown, 22 (17•8%) were members of the police and armed forces, 19 (15•4%) were intimate partners, and seven (5•8%) were relatives. Most women (30 [80%]) reported Boko Haram insurgents as perpetrators of physical violence followed by strangers (four women [12%]), men in the camp or host community (two women [5%]), and police and army (one woman [3%]). Overall, a third of women who experienced sexual violence (533 [33•0%]), and almost half those who reported physical violence (478 [48•0%]) sought care. We noted that 525 women (35•4%) and 511 women (37%) sought care for emotional and socioeconomic violence, respectively. Facility assessment showed that the basic facilities needed to manage sexual and gender-based violence and prosecute perpetrators were lacking in many health facilities and police stations in camps and host communities. Interpretation Interventions are urgently needed to improve the capacity of the health team and security personnel to better combat sexual and gender-based violence. Community-based committees for the prevention of such violence will also be of immense benefit in reducing the occurrence of the various forms of sexual and gender based violence. Fu...