The characteristics of people's relationships have relevance to health-high quality romantic relationships are associated with improved health whereas intimate partner violence is associated with poorer health. Recently, increased attention has been focused on the biological processes underpinning these associations. A geroscience approach-examining whether close relationship characteristics are associated with biological aging-would complement previous research focused on individual disease pathways. This study used participants from the Dunedin Study (N = 974) to investigate relationship characteristics and biological aging across almost 20 years, from age 26 to 45. Being involved in romantic relationships was associated with slower biological aging, β = -0.12, p < .001. This difference represented 2.9 years of aging over the two decades. Greater relationship quality was also associated with slower biological aging, β = -0.19, p < .001, whereas higher levels of partner violence were associated with faster biological aging, β = 0.25, p < .001. A 1 SD difference in these characteristics was associated with a difference of 1.0 and 1.3 years of aging over the two decades, respectively. Secondary analyses suggested that experiencing violence from a partner was more strongly associated with biological aging than perpetrating violence, and that the experience of physical violence was more strongly associated with aging than psychological violence. These findings suggest that the characteristics of romantic relationships have relevance for biological aging in midlife. Interventions designed to increase relationship quality and decrease partner violence could reduce future morbidity and early mortality by slowing people's biological aging. (PsycInfo Database Record (c) 2020 APA, all rights reserved).