This systematic review and meta-analysis aimed to investigate the effect of dietary-based lifestyle modification interventions (“diet,” or “diet + exercise,” or “diet + exercise + behavioral” intervention) on the measures of anthropometric and dietary intake parameters in women with breas cancer (BC). Databases were searched until June 2021. Inclusion criteria were randomized controlled trials that enrolled only women with BC. Studies that used exercise or behavioral interventions alone were not included. Mean ± SD changes were extracted for each outcome, and pooled using a random-effects model; 7315 studies were identified. Fifty-one studies (n = 7743) were included. The median ± SD duration of treatment was 24 ± 16.65 wk. Dietary-based interventions significantly reduced body weight [45 studies (n = 7239), weighted mean difference (WMD) (95% CI): −2.6 (−3.2, −2.1) kg], BMI [31 studies (n=5384); WMD (95% CI): −1.0 (−1.3, −0.7) kg/m2], lean body mass [15 studies (n= 1194); WMD (95% CI): −0.6(−0.7, −0.4) kg], fat mass [11 studies (n = 913); WMD (95% CI): –2.6 (−3.3, −1.8) kg], fat percentage [17 studies (n = 897); WMD (95% CI): −1.5 (−1.9, −1.3)%], hip circumference [9 studies (n= 489); WMD (95% CI): −2.43 (−3.34, −1.54) cm], and waist circumference [7 studies (n= 309); WMD (95% CI): 0.02 (−0.03, −0.005) cm]. Significant reductions in energy intakes [20 studies (n = 4608), WMD (95% CI): −162 (−220, 104) kcal/d] and fat intakes [7 studies (n = 4316), WMD (95% CI): −7.5 (−7.8, −7.2)% of energy/d], and an increase in fiber intakes [11 studies (n = 4241), WMD (95% CI): 2.4 (0.7, 4.1) g/d] were observed. No significant changes were seen in protein, carbohydrate, and fruit and vegetable intakes. Subgroup analyses showed that changes in anthropometric and dietary intake indices were significant in studies that enrolled patients with both obesity and normal weight, studies that used diet therapy in combination with exercise and behavioral therapy, and studies that started the intervention during the treatment period. Overall, a multimodal dietary-based lifestyle intervention had significant effects on anthropometric and dietary intake parameters in women with BC, specifically when started as early as the diagnosis. This meta-analysis was registered at PROSPERO as CRD42021291488. Adv Nutr 2022;13:1974–1988