Acquired brain injury (ABI) commonly causes cognitive dysfunction that needs to be assessed and treated to maximize rehabilitation outcomes. Research suggests that music, emotion, and cognition are intimately linked, and that music can contribute to the assessment and treatment of cognitive functions of adults who have suffered from ABI. To this date, no standardized music based assessment tool exists to identify and measure cognitive functioning and mood states of Spanish-speaking persons with ABI at treatment intake and over time. The objective of this study was to develop such a scale and determine its psychometric properties in terms of internal consistency, reliability, and concurrent validity. The “Evaluación de la Cognición Musical para Adultos con Lesión Cerebral Adquirida” (in English “Music Based Assessment for Cognitive Functions of Adults with Acquired Brain Injury – ECMUS”) was developed and tested at a neurorehabilitation institute in Argentina. Twenty-four healthy adults and 20 adults with ABI were recruited and assessed with the ECMUS. Despite the limited number of participants, this preliminary psychometric examination shows promising results. The tool has an acceptable internal consistency, excellent test–retest and inter-rater reliability, and, depending on the subscale, weak to strong correlations to related, nonmusical constructs. Overall, this pilot study opens the possibility to further explore the inclusion of music in assessment procedures of Spanish-speaking individuals with ABI in rehabilitation settings. It is clinically derived and emphasizes evidence-based contributions of the field of music therapy to interdisciplinary assessment and treatment in rehabilitation settings.