Despite the understanding that differentiation is a lifelong process crucial for psychological adaptation, there is limited knowledge regarding how parent–child differentiation in adulthood is associated with the psychological well-being of both parents and adult children. Furthermore, empirical research has yielded inconclusive results regarding whether the parental status of adult children influences the parent–child relationship. Consequently, the current study focuses on the moderating effect of adult daughters’ parental status on the association between aging mother–adult daughter differentiation and psychological well-being. The study utilized data from 167 pairs of Korean aging mothers and adult daughters to examine two main aspects: (1) the relationship between aging mother–adult daughter differentiation and psychological well-being; and (2) the moderating role of adult daughters’ parental status on the relationship between aging mother–adult daughter differentiation and psychological well-being. The findings revealed that both the differentiation of adult daughters and mothers was positively associated with their respective psychological well-being. However, no significant cross-interactional effects of aging mother–adult daughter differentiation on psychological well-being were observed. Notably, there was a positive moderating effect of the adult daughter’s parental status on the association between aging mother–adult daughter differentiation and psychological well-being for aging mothers.