Background: Weight gain and overweight have been related to an increased risk of recurrence and mortality in patients affected by EBC. However, the adherence to nutritional intervention is not entirely explored. The aims of this prospective study were to evaluate the ADG and the effectiveness of nutritional intervention in terms of BWC in pts with EBC undergoing treatment. Methods: Entry criteria: EBC pts candidate to neoadjuvant/adjuvant therapy. At study entry, pts received a nutrition evidence-based tailored educational intervention by a skilled dietitian. Anthropometric and dietary assessments were performed. ADG was estimated through the validated Med-Diet 14-item questionnaire. Health-Related Quality of Life was analyzed with the EORTC QLQ-C30. Descriptive statistics was adopted. Associations between variables and groups according to nutritional variables were analysed (Chi-square test). Results: From February 2016 to December 2018, 204 pts were enrolled (median age 49 years): 27.5%/72.5% neoadjuvant/adjuvant treatment. At baseline, 2.5% of pts were underweight, 41.7% were normal weight, 33.3% were overweight and 22.5% were obese. Moreover, 47.5% of pts gained ≥5% of their usual weight and 25.5% performed physical activity. Significant nutritional impact symptoms were reported: dyspepsia (51.5%), constipation (62.3%) and dysgeusia (34.8%). The majority of pts presented dietary patterns high in fat (median fat intake: 35.2% from total energy intake) and low in dietary fiber (median dietary fiber intake: 17.2 g/day). A significant correlation between baseline BMI and tension was observed (p < 0.0001) as well as BMI and worry, irritability and depression (p < 0.0001, p < 0.0001 and p = 0.008, respectively). Six months after the intervention, the median ADG was high (median Med-Diet score was 12). A high adherence to nutrition guidelines (defines as a Med-Diet score ≥10, 112 patients) significantly correlated with a weight loss ≥5% from the baseline weight (p = 0.005). Furthermore, the weight loss ≥5% was correlated with a lower rate of depression (p = 0.05). Conclusions: A tailored nutritional intervention for EBC pts undergoing adjuvant or neoadjuvant therapy has the potential to improve their ADG, in order to achieve a weight loss.