Obesity is associated with increased breast cancer recurrence and mortality. Though some post-diagnosis weight loss interventions have achieved weight loss outcomes, it is unclear whether they also improve diet quality. In the Lifestyle, Exercise, and Nutrition (LEAN) study, overweight or obese breast cancer survivors were randomized to either usual care group (n=33) or the 6-month lifestyle intervention (n=67). Dietary intake was assessed at baseline and 6 months using a validated food frequency questionnaire, and overall diet quality was calculated using the Healthy Eating Index (HEI)-2010 (range 0-100). Intervention effects on diet were evaluated with generalized linear models. Among the 81 participants (51 intervention, 30 usual care) with dietary data, the mean baseline HEI score was 70.5 (s.d.=8.8) and was improved at 6 months (intervention group=6.8 point increase vs usual care=3.1, P=0.09). Intervention group participants achieved greater reductions in percent of energy from total fat (-4.2% vs -1.2%; P=0.013) and saturated fat (-2.2% vs -1.1%; P=0.003), and greater increases in fiber (4.8 g per 1000 kcal vs 1.3 g per 1000 kcal; P=0.007) and fruit (0.5 servings vs 0.0 servings; P=0.006) intake. Intervention group participants who lost ⩾5% body weight (n=27) demonstrated significantly greater improvements in HEI score (10.4 vs 2.8) than those who lost <5% (n=23). The intervention increased fruit and fiber intake and decreased percent energy from fat, and those with greater weight loss achieved greater increases in overall diet quality. These findings support the ability of a weight loss intervention to improve diet among breast cancer survivors.