Some cases of asymptomatic traumatic cyst can be sizable; therefore, they require complete curettage and grafting with bone substitution materials. This case report presents a sizeable traumatic mandibular cyst in a young man treated by surgical exploration and filled with autologous dentin graft (ADG) prepared from an extracted impacted tooth 48 (FDI tooth-numbering system) and advanced platelet-rich fibrin (A-PRF). Initially, an A-PRF membrane was used to cover the apices of teeth 42 and 43, which were protruding into the defect to protect their periapical structures. Then, a grafting strategy was introduced to achieve two fronts of bone formation: one by stimulation of bone outgrowth from the periphery due to A-PRF cellular activity, and a second by bone deposition directly on dentin particles in the center of the defect. On CBCT scans performed 7 months postoperatively, arrays of trabeculae that were extending from bone boundaries of the cyst defect were merged with more condensed bone deposited on ADG residuals in the center, thus filling the defect. It was found that autologous dentin combined with cellular A-PRF activity is a powerful tool to restore even sizable bone defects in a relatively short time frame with adequate bone remodeling.