Background <p>The increasing importance of sexual expression among older Europeans calls for a more comprehensive understanding of the process of sexual aging. Combining social gerontological and sexological theories, the current study aimed to conceptualize and operationalize a process-focused model of successful sexual aging (the SSAM).</p> Methods <p>Using a Croatian sample of older partnered individuals (<i>n</i> = 200; <i>M</i>_age = 71.2) and a large-scale German sample of both partnered (<i>n</i> = 442; <i>M</i>_age = 69.2) and non-partnered individuals (<i>n</i> = 971; <i>M</i>_age = 69.2), we developed and validated a 9-item measure of successful sexual aging.</p> Results <p>The 3-dimensional Successful Sexual Aging Scale (SSAS) reflects two internal processes (acceptance of and adaptation to aging-related changes) and an external one (opportunities for sexual expression). The measure was age, gender, and (to a large extent) culture invariant in older partnered individuals. With a minor adjustment (re-wording of a single item), the SSAS assessed positive sexual aging comparably well in non-partnered individuals. As hypothesized, the measure was significantly related to positive attitudes about older people’s sexuality, life and relationship satisfaction, subjective age, and self-reported health. SSAS scores, which were lower among Croatian than German participants, were also linked to lower levels of social isolation, as well as depression and anxiety symptoms.</p> Conclusions <p>The new measure can be useful in a range of quantitative assessments of older people’s sexuality and well-being. Furthermore, the concept of successful sexual aging may assist in challenging ageism and in interventions focusing on positive sexual aging.</p> Clinical trial number <p>Not applicable.</p>