Abstract Background Few studies have examined the role of maternal stress in relation to their children’s dietary quality and its trajectory over time. Purpose The objective of this longitudinal study was to examine the effect of baseline maternal stress on the change in their 8- to 12-year-old children’s dietary quality over 1 year. Methods Mother–child dyads (N = 189) from the greater Los Angeles area participating in the Mothers’ and their Children’s Health (MATCH) study in 2014–2016 completed assessments at baseline, 6 months, and 12 months. At baseline, mothers (mean age = 41.0 years, standard deviation [SD] = 6.1) completed the 10-item Cohen’s Perceived Stress Scale (PSS). At each time point, children (51% female, mean age = 9.6 years, SD = 0.9) completed up to two 24-hr dietary recalls. Dietary data were used to calculate each child’s Healthy Eating Index 2010 (HEI-2010) score at each time point. Multilevel models examined the effect of time on the patterns of change in children’s HEI-2010 scores over 1 year and the cross-level interaction between baseline maternal PSS score and time on the change in children’s HEI-2010 scores. Results On average, there was no significant linear change in child HEI-2010 across 1 year (b = −0.410, p = .586). Controlling for covariates, the rate of change in HEI-2010 differed depending on mother’s baseline PSS (i.e., significant cross-level interaction effect) (b = −0.235, p = .035). Conclusions Our results showed that higher-than-average maternal stress at baseline was associated with greater decline in children’s dietary quality over 1 year. Family-based dietary interventions that incorporate maternal stress reduction could have positive effects on children’s dietary quality.