Introduction: Fat graft (FG) is widely used in breast reconstructive surgery (RS) following mastectomy or lumpectomy for breast cancer (BC); however, concerns persist about its oncological safety. This study evaluates the impact of FG reconstruction compared to other techniques on BC survival in young women. Methods: We identified patients aged 18-45 treated for non-metastatic BC on the French National Healthcare System Database between January 1, 2010, and December 31, 2018. Patients undergoing FG were matched with patients undergoing other RS based on BC subtype, chemotherapy, radiotherapy, and RS timing after BC surgery. Analyses were stratified on breast surgery (mastectomy or lumpectomy). Results: A total of 50,178 patients were included: 28,260 with luminal tumors, 7,982 with TNBC, 7,087 with HER2-positive tumors, and 6,849 with undefined tumors. In the mastectomy and lumpectomy group, 7,333 and 883 patients respectively received FG. In the mastectomy group, after matching, FG was associated with better Disease-free survival (DFS) (HR 0.76, 95%CI 0.65-0.88) and Overall Survival (OS) compared to patients with other RS. FG also showed improved DFS (HR 0.59, 95%CI 0.51-0.69) compared to patient without any RS. No significant differences in DFS or OS were found in the lumpectomy group. Conclusion: FG appears to improve survival outcomes in young women undergoing mastectomy with or without additional RS. In contrast, no significant impact was observed in the lumpectomy group. These findings suggest that FG can be a safe reconstructive option, particularly after mastectomy. Further research is needed to confirm these results and understand the underlying mechanisms.