Engaging in a romantic relationship represents one important life experience in young adulthood that has been shown to catalyze age-related decrease in neuroticism (Neyer & Lehnart, 2007). The current research builds directly on this finding by investigating one process that underlies the partnership effect. We focused on the relationship-specific interpretation bias (RIB; Finn, Mitte, & Neyer, 2013), which is the tendency to interpret ambiguous partner and relationship scenarios in a negative way. It was expected that the RIB decreases within relationships in young adulthood and that this decrease in turn predicts long-term declines in neuroticism. A sample of 245 young adult romantic couples was assessed four times across 9 months. Actor and partner effects of changes in the RIB on changes in neuroticism were analyzed using a dyadic dual change model. Recent time-to-time decreases in the RIB predicted one's own (actor effect) decline in neuroticism across 9 months. Similarly, there was a trend for a partner effect. We conclude that changes in biased relationship-specific interpretations reflect one unique process that contributes to the understanding of romantic relationship effects on personality development.