Loneliness is a subjective measure of one’s state of mind and the negative feelings about one’s level of social contact, which usually involves an unwanted discrepancy between existing and desired relationships. It is measured through self-reported questionnaires, among which a widely used one is the de Jong Gierveld Loneliness Scale (DJGLS). Although it has been previously tested and proven to be a reliable tool for measuring loneliness of Spanish older people, some relevant research questions remain unsolved. Therefore, this article aims to present a validity study of the Spanish version of the DJGLS for its use with older people, by establishing its factorial validity; studying the adequateness of a Rasch model underlying participants’ responses; offering evidence of its reliability; and testing for convergent and discriminant validity. Data were obtained in a survey conducted with older adults attending Lifelong Learning programs in Valencia (Spain). The sample consisted of 335 people aged 55 years or older. Sociodemographic data were collected, and loneliness was measured with the DJGLS and the UCLA Loneliness Scale. Results have supported a unidimensional substantive structure, but with minor method effects associated with negatively worded items. In terms of multidimensional IRT analysis, the 2 Parameters Logistic Model fitted the data well and offered better information functions than the Rasch model. Reliability and criterion-related validity estimates were adequate.