Personality traits are strong predictors of subjective well-being with meta-analyses supporting the relationships. However, the role of personality similarity in romantic couples, especially among older adults, remains unclear. This article aims to examine the actor and partner effects of personality traits on subjective well-being among older people in Europe. They include the effects on longitudinal development of subjective well-being as well as the under-researched impact of partners’ (dis)similarity in terms of personality traits. The study analyzes subjective well-being among older European couples using data from three waves of the Survey of Health, Aging and Retirement in Europe (from 2017-2022). It employs APIM and DRSA models to examine actor, partner, and personality similarity effects. Subjective well-being is measured using the CASP-12 scale, while personality traits are assessed using the Big Five inventory. Personality predicted subjective well-being over time at actor and partner levels. Neuroticism showed the strongest negative, conscientiousness the strongest positive actor effect. Extraversion had the strongest partner effect. Actor effects were generally stronger than partner effects for subjective well-being levels, but the reverse was true for subjective well-being development. Similarity effects were limited, though trait combinations revealed nuanced interactions. The findings confirm the importance of both partners’ personality traits for long-term subjective well-being, with effects varying by trait and outcome. They underscore the value of a dyadic longitudinal approach in revealing complex, trait-specific dynamics beyond simple similarity effects.