Findings from research assessing the influence of dietary factors on child obesity have been equivocal. We aimed to test the hypothesis that a positive change in diet quality is associated with favourable changes in BMI z-scores in schoolchildren from low socio-economic backgrounds; and to examine whether this effect is modified by BMI category at baseline. This study utilized data from a subsample (n=216) of the Resilience for Eating and Activity Despite Inequality (READI) study, a longitudinal cohort with data collected in 2007-08 (T1) and 2010-11 (T2) in socio-economically disadvantaged women and children (5-12 years at T1). Dietary data was collected using a food frequency questionnaire, and diet quality index (DQI) scores derived at both times. Objective measures of weight, height and physical activity (accelerometers) were included. The other variables were reported in questionnaires. We examined the association between change in DQI and change in zBMI, with linear regression analysis adjusted for physical activity, screen sedentary behaviour and maternal education, both in the whole sample, and stratified by overweight status at baseline. After accounting for potential covariates, change in diet quality was inversely associated with change in zBMI only in children who were overweight at baseline (P=0.035), thus supporting the hypothesis that improvement in diet quality is associated with a concurrent improvement in zBMI among already overweight children, but not those of normal BMI status. The identification of modifiable behaviours such as diet quality that affect zBMI longitudinally is valuable to inform future weight gain prevention interventions in vulnerable groups.