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Description: Generational differences in personality are a widespread assumption in society and developmental theory. Yet evidence for historical change in adult Big Five traits is scarce for levels and particularly developmental change. We tracked adult trajectories of personality of 4,732 participants (Mage = 52.93, SDage = 16.69, 53% female) in the Seattle Longitudinal Study (born 1883 thru 1976) across 50 years. Multi-level models revealed evidence for historical change in personality: At age 56, later-born cohorts exhibited lower levels of maturity-related traits (agreeableness and neuroticism) and higher levels of agency-related traits (extraversion and openness) than earlier-born cohorts. Historical changes in agreeableness and neuroticism were more pronounced among young adults and in openness less pronounced. Cohort differences in change were rare and only observed for agreeableness, with more pronounced within-person increases among later-born cohorts. Our results yield first evidence for historical change in the Big Five across adulthood and point to the roles of delayed social investment and maturity effects.