OBJECTIVE: This study examined whether social activity diversity, a novel concept indicating an active social lifestyle, is associated with lower subsequent loneliness, and decreased loneliness is further associated with less chronic pain over time. METHODS: 2528 adults from the Midlife in the United States Study (Mage = 54 yrs) provided data at baseline (2004-2009) and 9 years later. Social activity diversity was operationalized by Shannons entropy that captures the variety and evenness of engagement across 13 social activities (0-1). Participants reported feelings of loneliness (1-5), presence of any chronic pain (yes/no), the degree of chronic pain-related interference (0-10), and the number of chronic pain locations. Indirect associations of social activity diversity with chronic pain through loneliness were evaluated, adjusting for sociodemographics, living alone, and chronic conditions. RESULTS: Higher social activity diversity at baseline (B = -0.21, 95%CI = [-0.41, -0.02]) and an increase in social activity diversity over time (B = -0.24, 95%CI = [-0.42, -0.06]) were associated with lower loneliness 9 years later. An increase in loneliness was associated with 24% higher risk of any chronic pain (95%CI = [1.11, 1.38]), greater chronic pain-related interference (B = 0.36, 95%CI = [0.14, 0.58]), and 17% increase in the number of chronic pain locations (95%CI = [1.10, 1.25]) at the follow-up, after controlling for corresponding chronic pain at baseline and covariates. Social activity diversity was not directly was associated with chronic pain, but there were indirect associations through its association with loneliness. CONCLUSION: Diversity in social life may be associated with decreased loneliness, which in turn, may be associated with less chronic pain, two of the prevalent concerns in adulthood.