Background and Objectives: Surgical site infection (SSI) is a significant complication of non-reconstructive and reconstructive breast surgery. This study aimed to assess SSI after breast surgery over five years in a single center in Poland. The microorganisms responsible for SSI and their antibiotic susceptibilities were determined. Materials and Methods: Data from 2129 patients acquired over five years postoperatively by the Department of Surgical Oncology, Medical University of Gdansk in Poland were analyzed. Results: SSI was diagnosed in 132 patients (6.2%) and was an early infection in most cases (65.2%). The incidence of SSI was highest in patients who underwent subcutaneous amputation with simultaneous reconstruction using an artificial prosthesis (14.6%), and breast reconstruction via the transverse rectus abdominis muscle (TRAM) flap method (14.3%). Gram-positive bacteria were responsible for SSI in most cases (72.1%), and these were mainly Staphylococcus strains (53.6%). These strains were 100% susceptible to all beta-lactam antibiotics (except penicillin) but were less susceptible to macrolides and lincosamides. Conclusions: SSI is a serious problem, and attention should be focused on its prevention. Reconstruction using an artificial prosthesis or via the TRAM flap method is connected to increased SSI incidence. Further studies are required to prevent SSI following breast surgery.