Purpose The purpose of this paper is to assess the prevalence of depression and post-traumatic stress disorder (PTSD) among internally displaced persons (IDPs) in Maiduguri. Design/methodology/approach The study was a cross-sectional study that took place among the six IDP camps located in Maiduguri metropolis in Borno State. A non-randomized technique was used to sample 1,200 respondents. Face-to-face interviews with selected members of households were carried out confidentially. The study used the Patient Health Questionnaire (PHQ-9) and Impact of Event Scale-6 which were translated to Kanuri. Descriptive and inferential statistics were employed using SPSS version 21. Findings The response rate was 100 percent. In total, 96.1 percent (1,153) of the respondents were depressed, while 78 percent (936) of the respondents were symptomatic for PTSD. The prevalence rate of comorbid PTSD with depression was 68.1 percent (817). About one-third of the respondents had moderately severe depression (29.6 percent, n=355) while about one in ten of them were severely depressed (11.3 percent, n=136). The odds of being depressed was 3.308 higher in people aged 51–60 years compared to people between 18 and 20 years. Significant predictors of depression in the sampled population were screening positive for PTSD and being unemployed. Practical implications The high prevalence of depression and PTSD among the sampled population calls for structured interventions to deal with mental health problems. The study findings suggest the need for more research (preferably qualitative) on the mental health issues in this population. Originality/value This study contributes to the sparse available literature on the mental health of IDPs in Nigeria.