Introduction: Breast cancer is the most commonly diagnosed malignancy in women and the leading cause of cancer death in women. Tumor size is a critical factor in determining the type and extent of surgical and oncologic treatment. It is accurately determined by imaging modalities such as mammography, ultrasound, and magnetic resonance imaging (MRI), which provide a more reliable determination of tumor size. The aim of our study was to investigate the impact of preoperative breast magnetic resonance imaging on surgical treatment of newly diagnosed breast cancer. Material and Methods: The study retrospectively reviewed the records of 241 participants with newly diagnosed breast cancer who underwent preoperative mammography, breast ultrasound, and MRI between 2016 and 2020 at University Hospital Centre Rijeka. Patients were diagnosed with invasive ductal carcinoma, invasive lobular carcinoma, ductal carcinoma in situ, or a combination of the types. Surgical treatment included one of the following procedures: simple quadrantectomy, quadrantectomy and sentinel lymph node biopsy, quadrantectomy and axillary lymph node dissection, mastectomy and sentinel biopsy, or mastectomy and axillary dissection. Results: Compared with histopathologic tumor size, breast MRI overestimated size in 10% of patients. T stage was underestimated in 5% of patients (p>0.050). In comparison, breast ultrasound overestimated tumor size in 12% and underestimated it in 48% (p<0.001). Similarly, mammography overestimated tumor size in 14% and underestimated it in 62% (p<0.001). Conclusion: In patients with newly diagnosed breast cancer, the use of preoperative breast MRI as an adjunct to mammography and ultrasound for locoregional staging significantly alters subsequent surgical treatment.