Background: The detection rate of Multifocal/Multicentric Breast Cancer (MMBC) is increasing, garnering greater clinical attention. However, prognostic differences between MMBC and Unifocal Breast Cancer (UBC) remain controversial. Methods: This retrospective cohort study analyzed 109 MMBC and 4,198 UBC patients who underwent mastectomy at the Breast Center of the Fourth Hospital of Hebei Medical University from 2010 to 2014. Propensity score matching (PSM, 1:2 ratio, caliper = 0.1) balanced baseline characteristics. Two T-staging methods were evaluated: Tmax (largest lesion diameter) and Tsum (cumulative lesion diameters). Outcomes included Locoregional Recurrence (LRR), Disease-Free Survival (DFS), and Overall Survival (OS). Results: MMBC patients were younger (median 46 vs. 52 years, p < 0.001), more frequently premenopausal (67.0% vs. 48.5%, p < 0.001), and had higher lymph node involvement (48.6% vs. 37.1%, p < 0.001). Under Tmax staging, MMBC showed comparable OS (p = 0.127) and LRR (p = 0.118) but inferior DFS (p = 0.010) versus UBC. Tsum staging increased TNM stages in 45.0% of MMBC patients, aligning DFS (p = 0.079) and OS (p = 0.269) with UBC. Multivariate analysis identified MMBC (HR: 0.387, p = 0.015) and nodal involvement (HR: 2.446, p = 0.022) as independent DFS predictors in Tmax-staged patients. Conclusions: Tsum staging better reflects MMBC tumor burden, mitigating prognostic disparities observed under Tmax staging. Current Tmax-based protocols may underestimate MMBC severity, advocating for revised staging criteria.