This study evaluated the validity and reliability of the Montessori Assessment System. The Montessori Assessment System assesses preserved abilities in persons with moderate to severe dementia. In this respect, this instrument provides crucial information for the development of effective person-centered care plans. A total of 196 persons with a diagnosis of dementia in the moderate to severe stages of dementia were recruited in 10 long-term care facilities in France. All participants completed the Montessori Assessment System, the Clinical Dementia Rating Scale and/or the Mini Mental State Examination and the Severe Impairment Battery-short form. The internal consistency and temporal stability of the Montessori Assessment System were high. Additionally, good construct and divergent validity were demonstrated. Factor analysis showed a one-factor structure. The Montessori Assessment System demonstrated satisfactory psychometric properties while being a useful instrument to assess capabilities in persons with advanced stages of dementia and hence to develop person-centered plans of care.