Naturalistic online grocery stores could provide a novel setting for evaluating nutrition interventions. In 2021–2022, we recruited US adults (n 144, 59% low-income) to complete two weekly study visits: one in a naturalistic (‘mock’) online grocery store developed for research and one in a real online grocery store. Participants selected groceries and responded to survey questions. Analyses examined survey responses and expenditures on fifteen food categories (e.g., bread, sugar-sweetened beverages). Nearly all enrolled participants completed both visits (98% retention). Moreover, nearly participants all reported that their selections in the naturalistic store were similar to their usual purchases (95%) and that the naturalistic store felt like a real store (92%). Participants’ spending on food categories in the naturalistic store were moderately-to-strongly correlated with their spending in the real store (range of correlation coefficients: 0⋅36–0⋅67, all P-values < 0⋅001). Naturalistic online grocery stores may offer a promising platform for conducting nutrition research.