Nightmares have been shown to be robust predictors of self-harm risk, beyond depressive symptoms and hopelessness at times. However, few studies have investigated associations between nightmare content and increased self-harm risk. This study explored associations of thematic nightmare content with history of self-harm, and risk of self-harm phenomena the morning following a nightmare. A mixed-method diary study was performed. Prospective nightmare reports were obtained from 72 participants. A total of 47 nightmare reports met inclusion criteria and were analyzed for themes using inductive thematic analysis. Chi-square and bootstrap Pearson’s correlation tests were performed to assess the associations between nightmare themes and self-harm history, and risk of self-harm phenomena following a nightmare. “Powerlessness to Change Behavior” was associated with a history of self-harm engagement, whereas “Financial Hardship” indicated reduced risk. Themes were not significantly associated with increased risk of self-harm phenomena following a nightmare. Content may be of use in detecting lifetime history of self-harm engagement, particularly in populations where disclosure is seen as taboo. However, nightmare symptom severity remains a better indicator of risk. Evidence for the utility of nightmare content in assessing immediate self-harm risk is presently lacking. Replication with increased power is recommended.