BACKGROUND: Mobility assessments are commonly used among older adults as risk stratification for falls, preoperative function, frailty, and mortality. We determined if gait speed and self-reported difficulty walking are similarly associated with social isolation and loneliness, which are key markers of social well-being and linked to health outcomes. METHODS: We used 2015-2016 data from the National Social life Health and Aging Project (NSHAP), an in-person nationally-representative survey of 2640 community-dwelling adults ≥65 years old. We measured gait speed (timed 3-m walk: unable to walk, ≥5.7 s, and <5.7 s), and self-reported difficulty walking one block or across the room (unable, much, some, or no difficulty). Social measures included loneliness (3-item UCLA scale), social isolation (12-item scale), and individual social activities (frequency socializing, religious participation, community participation, and volunteering). We used logistic regression to determine the adjusted probability of each social measure by gait speed and difficulty walking, adjusting for sociodemographic and health characteristics, and tested for interaction terms with age. RESULTS: Participants were on average 75 years old (SD = 7.1), 54% female, 9% Black/African American, and 6% Hispanic. Difficulty walking one block was associated with (p < 0.05): social isolation (much difficulty: 26% vs no difficulty: 18%), low socializing (33% vs 19%), low volunteering (67% vs 53%), low community participation (54% vs 43%), low religious participation (51% vs 46%), and loneliness (25% vs 14%). Difficulty walking across the room was similarly strongly associated with social isolation and individual activities. The association between self-reported difficulty walking and social isolation was stronger at older ages (p-value of interaction <0.001). CONCLUSIONS: Self-reported mobility difficulty is a widely used clinical assessment that is strongly associated with loneliness and social isolation, particularly at older ages. Among persons with limited mobility, clinicians should consider a careful social history to identify social needs and interventions addressing mobility to enhance social connections.