Background: Certain lifestyle factors affect the risk of developing breast cancer. Especially, diet and physical activity play a primary role in preventing breast cancer. However, the results of studies on this subject in different societies are still conflicting. Objective: The objective of this study is to determine the effects of dietary habits and sedentary lifestyle on breast cancer risk among women attending the Oncology Day Treatment Center at a state university in Turkey. Materials and Methods: This case-control study was conducted in the Oncology Day Treatment Center at a state university in Turkey between December 1(st), 2016 and June 1(st), 2017. The case group consists of 65 women diagnosed with breast cancer, and the control group consists of 65 women without any chronic disease. The Mediterranean diet score was used to assess the dietary habits of the participants, and the International Physical Activity Questionnaire Short Form was used to assess the physical activity (sedentary lifestyle) of the participants. The ShapirouWilk W test was used to check for normality within the distribution of scale scores. Categorical data were compared using the Chi-square test. Multivariate binary regression analysis was conducted (P < 0.05). Results: The proportion of participants who received a higher score from the Mediterranean diet score was significantly lower in the case group than in the control group. The proportion of physically inactive individuals in the case group was higher than those in the control group. Body mass index (BMI) at the age of menopause was significantly higher in the case group than the control group. Those who frequently use a deep-frying cooking method to cook red meat have a 6.77 times higher breast cancer risk than those who use a stewing method. Comparing those who do not consume olive oil, or who consume olive oil rarely, once or twice a week compared with those who consume olive oil daily, it was found that the case group has 4.5 times higher risk than the control group. Conclusions: Cooking red meat by a deep-frying method, lack of physical activity, having a higher BMI particularly during the postmenopausal period, and nonadherence to the Mediterranean diet are risk factors of breast cancer.