: To quantitatively assess the long-term volume stability of acellular dermal matrix (ADM) in oncoplastic breast-conserving surgery (OBCS) and analyze surgical and aesthetic outcomes. : This retrospective study examined 172 breast cancer patients who underwent OBCS with immediate ADM-based volume replacement (2020-2022). Patients received either diced ADM with sheet ADM ( = 102) or diced ADM with paste ADM (n = 70). The ADM volume was evaluated using MRI at 6, 12, and 24 months postoperatively. : Long-term volume stability was achieved in both groups with minimal volume reduction (4.3-4.5%) at 24 months ( < 0.001). Early surgical complications included hematoma (4.1%), seroma (2.3%), and wound issues (1.2%), with no infections or ADM non-incorporation. Contour irregularities occurred in 16.3% of cases. Radiotherapy (87% of patients) did not significantly impact ADM volume retention. : ADM provides predictable, durable volume replacement in OBCS, with excellent volume stability even with radiotherapy. This quantitative assessment of ADM volume retention over two years supports ADM as a reliable option for breast-conserving surgery, potentially expanding treatment options for patients with unfavorable tumor-to-breast volume ratios.