In geriatric psychiatry, informant reports are often important due to cognitive problems and related impaired insight and judgment. Informant questionnaires to identify personality traits among older adults are sparse. The Dutch informant personality questionnaire (the HAP) is especially developed to address this need. The objective of this study is the psychometric evaluation of the HAP among older adults in the Netherlands and Belgium. We investigated the internal consistency, gender differences, the test-retest and inter-rater reliability, the factorial structure, and the concurrent validity. Informants completed the HAP ratings of nursing home residents (n = 385) and elderly psychiatric patients (n = 204). The internal consistency of the scales is good. Medium gender differences on three scales were found in the population Psychiatry. The inter-rater and test-retest reliability are good to excellent. There are significant similarities between a number of HAP scales and dimensions of the Big Five. The congruence between the factor structures in both samples is very high. We labeled the three factors externalizing/antagonistic, internalizing/neurotic, and compulsive. The HAP meets the need for validated and reliable informant instruments for personality assessment among older adults in geriatric psychiatry. The content scales of the questionnaire address traits of the premorbid personality. Therefore, the HAP might be useful for personality assessment and selecting treatment options in mental healthcare and can be applied in scientific research in the area of personality aspects in late life.