Studies suggest that loneliness is associated with age. Among older adults, women and Black adults may be at greater risk than men and White adults, respectively. Social and physical contexts are also linked with loneliness. However, little is known about whether and how those of different genders and racial/ethnic groups may experience social and physical contexts differently in terms of their real-time loneliness, and the extent to which these differences may be explained by differential exposure or reactivity to such contexts. We examine (1) how momentary loneliness relates to (a) gender and race/ethnicity and (b) social and physical context; and the extent to which gender and racial/ethnic groups may be (2) differentially exposed to loneliness-related contexts and/or (3) differentially reacting to these contexts. Using multilevel regressions, we analyzed ecological momentary assessments from 342 community-dwelling U.S. older adults from the Chicago Health and Activity Space in Real Time study. In each of three waves of data collection, smartphone "pings" (five per day for 21 days; n = 12,744 EMAs) assessed loneliness, social context (e.g., alone, with a spouse/partner), and location/physical context (e.g., home, at work). Results revealed that men consistently reported greater loneliness intensity than women, including after adjusting for momentary physical and social context. In addition, those momentarily outside the home and/or not alone were less likely to feel lonely than their counterparts. However, the protective effect of being outside of the home (vs. home) was weaker among women and Black and Hispanic older adults, and the protective effect of being with one or more others (vs. alone) was weaker among women. Results are among the first to identify contextual effects on real-time loneliness in older adults and how these associations vary by gender and race/ethnicity. Knowledge regarding momentary variation in loneliness may inform future just-in-time adaptive loneliness interventions in older adulthood.