SummaryThis psychometric pilot study aims to evaluate a new multidimensional simple scale, named the nightmare severity index (NSI) – close to the existing insomnia (ISI) and hypersomnia (HSI) severity indexes. The NSI encompasses all main dimensions of nightmare disorder, evaluating four subdimensions: frequency, emotional impact, diurnal impact, and nocturnal impact of nightmares. The NSI was completed by a total of 102 patients. The majority of the population consisted of women (64%) and outpatient individuals (76%) diagnosed with mood disorders such as depression (31%) and bipolar disorder (41%). Comorbidity with post‐traumatic stress disorder (PTSD) was prevalent (44%), and psychotropic medications were commonly used (47%). Internal validity analyses indicated that the NSI was well suited for exploratory factor analysis. All items demonstrated satisfactory correlations with the factors, and the questionnaire exhibited good internal consistency (Cronbach's alpha >0.7). Higher NSI scores were observed among individuals experiencing nightmare symptoms considering the DSM‐5/ICSD‐3 criteria. In summary, the NSI proves to be a promising and valuable tool for clinical practice, demonstrating good acceptability, internal validity, and the ability to assess nightmare severity.