The current study aimed to conceptually replicate previous studies on the effects of actor personality, partner personality, and personality similarity on general and relational well-being by using response surface analyses and a longitudinal sample of 4,464 romantic couples. Similar to previous studies using difference scores and profile correlations, results from response surface analyses indicated that personality similarity explained a small amount of variance in well-being as compared with the amount of variance explained by linear actor and partner effects. However, response surface analyses also revealed that second-order terms (i.e., the interaction term and quadratic terms of actor and partner personality) were systematically linked to couples’ well-being for all traits except neuroticism. In particular, most response surfaces showed a complex pattern in which the effect of similarity and dissimilarity on well-being depended on the level and combination of actor and partner personality. In addition, one small but robust similarity effect was found, indicating that similarity in agreeableness was related to women’s experience of support across the eight years of the study. The discussion focuses on the implications of these findings for theory and research on personality similarity in romantic relationships.