Background Overweight is the common issue existed among breast cancer patients, leading to poor cancer prognosis and survival. However, previous studies have focused merely on single nutrient or food groups. This study aimed to determine whether diet quality was associated with overweight and laboratory parameters among breast cancer patients undergoing treatment. Methods A cross sectional study comprises of 163 patients was conducted from January to May 2017. Diet quality and level of physical activity of patients were ascertained using Healthy Eating Index (HEI)-2015 and International Physical Activity Questionnaire (IPAQ)-short form, respectively. Data were available on body mass index and biochemical data, including of serum albumin, serum hemoglobin and neutrophils count. Logistic regression was performed to identify the association between diet quality and overweight, serum albumin, serum hemoglobin and neutrophils count. Results The mean total HEI score (standard deviation) of the breast cancer patients was 63.86 (8.75). More than 80% of the subjects did not adhere to the dietary guidelines for whole grains, dairy, refined grains, and ratio of poly- and mono-unsaturated fatty acids to saturated fatty acids components. In the multivariate model, those who perceived higher score of total HEI-2015 were less likely to experience overweight (Q4 vs Q1: OR=0.36; 95% CI=0.13, 0.99) while reported higher concentration of serum hemoglobin (Q4 vs Q1: OR=0.36; 95% CI=0.13, 0.98; Q3 vs Q1: OR=0.35; 95% CI=0.13, 0.92). Non-significant associations of diet quality were shown for serum albumin and neutrophils count. Conclusions A majority of breast cancer patients require dietary modification, especially for fatty acids, grains and dairy products. Diet quality may associate with overweight. A balanced diet that address the problem of overweight may help to maintain a good nutritional status and to improve cancer care in breast cancer patients during treatment. Future prospective studies are required to confirm this association.