Self-harm is a serious public health concern in New Zealand and overseas due to its association with suicide and increasing prevalence rates. The prison population is known to be especially vulnerable to self-harm, with prisoners being 2-5 times more likely to self-harm compared to the general population. However, there is limited research available on how to identify at-risk prisoners. The current study aims to provide up-to-date prevalence rates of self-harm incidents across New Zealand prisons and highlight significant individual- and unit-level risk factors. This study is the first to use mixed-effects modelling to identify both individual- and unit-level risk factors that increase the likelihood of prisoners engaging in self-harm and its subcategories: threatening self-harm, non-life-threatening self-harm, life-threatening self-harm, and suicide. Two models were developed, the first included individual-level factors and unit as a random effect, the second also included unit-level fixed effects. Prisoners were at risk of self-harming the longer they spent in prison, not sharing a cell, were of European ethnicity, younger, had previous violent offences, and were in a high security unit and in units with multiple, opposing gangs in it. Prisoners in units with a high percentage of members from the same gang were less likely to self-harm. These identified predictive factors act as potential targets for risk assessments and treatment programmes to help reduce the risk of prisoners self-harming while incarcerated.