INTRODUCTION: Supermarkets are a major source of food for many families. Knowledge of how educational attainment affects the relationship between in-store environments of supermarkets and diet is needed. This study examined the relationship between maternal dietary quality and overall in-store supermarket environment, and assessed the effect modification of educational attainment. METHODS: Dietary quality z-scores were calculated for 829 mothers with young children using cross-sectional data collected in 2010-2011 from a 20-item food frequency questionnaire. Information about nine in-store factors (variety, price, quality, promotion, shelf placement, store placement, nutrition information, healthier alternatives, and single fruit sale) on 12 foods known to discriminate between better and poorer dietary quality were collected to create a standardized "healthfulness" z-score for each supermarket where mothers shopped. RESULTS: Multilevel unadjusted linear regression analysis completed in 2014-2015 showed that shopping at more-healthful supermarkets was associated with better dietary quality (β=0.39 SD/SD, p=0.01, 95% CI=0.10, 0.68). However, the relationship differed according to educational attainment (interaction, p=0.006). Among mothers who left school at age 16 years, those who shopped at less healthful supermarkets had poorer dietary quality (β=0.31 SD/SD, 95% CI=0.07, 0.55). Among mothers with degrees, those who shopped at less healthful supermarkets had better dietary quality (β=-0.59 SD/SD, 95% CI=-1.19, 0.00). CONCLUSIONS: Mothers with low educational attainment show greater susceptibility to less healthful in-store environments than mothers with higher educational attainment who may be protected by greater psychological and financial resources. Policy initiatives to improve supermarket environments may be necessary to address dietary inequalities.