No review has specifically focused on the experience of nightmares in individuals with a mental disorder. With a better understanding of nightmares in this population, clinicians will be more inclined to investigate for the presence of chronic nightmares, to consider nightmares for prognosis, and to treat this sleep difficulty independently from other mental disorders. Therefore, this narrative review aims to summarize the most relevant literature on the experience of nightmares in posttraumatic stress disorder (PTSD), depressive disorders and bipolar disorders, anxiety disorders and obsessive-compulsive disorder, attention-deficit/hyperactivity disorder, schizophrenia spectrum disorders, substance use disorders, autism spectrum disorder, eating disorders, and personality disorders. Differences in the experience of nightmares between mental disorders are also addressed. Expectedly, the positive relationship between nightmares and PTSD is the most empirically supported. Empirical data generally support a positive relationship between nightmares and other mental disorders, with the autism spectrum disorder being an exception. Moreover, the presence of nightmares in individuals with a mental disorder is often associated with poorer mental health, poorer sleep, and a greater risk for suicide. In conclusion, this review highlights the importance for clinicians to investigate for the presence of chronic nightmares along with other sleep difficulties (most commonly, insomnia and sleep apnea), to consider the potential influence of nightmares on the course of the primary mental disorder, and to be prepared to grant access to treatments targeting nightmares.