Bone transport and vascularized bone grafts are standard therapies for massive bone defects due to severe trauma and bone resection in osteomyelitis patients. Bone regeneration therapy using growth factors and scaffolds has been attracting attention. This study assesses the efficacy of a gelatin p-tricalcium phosphate (p-TCP) sponge combined with platelet-rich plasma (PRP) for treating massive bone defects in rabbits. Massive bone defects created in the ulnar diaphyses of 30 Japanese white rabbits were treated with either gelatin p-TCP sponge and PRP (group A; n = 10) or gelatin p-TCP sponge alone (group B; n = 10), or received no treatment (group C; n = 10). Radiographic examinations were performed weekly, and Lane-Sandhu X-ray scores were used to compare the groups. Eight weeks after bone defect creation, microcomputed tomography (pCT), bone mineral densitometry, and histological examinations were performed. Radiographs showed that the defects in group A were occupied by new bone, unlike in the other two groups. The Lane-Sandhu X-ray score for group A was significantly higher than those for the other groups starting at an early stage. Three-dimensional pCT images showed that the defects in group A were filled by new bone. Cross-sectional pCT images did not show radial-ulnar fusion at either end of the bone defects in group A, whereas radial-ulnar fusion was observed in the other two groups. Bone mineral density in group A was significantly higher than those in the other groups. Histological images from group A showed continuity of cortical bone with medullary cavity formation at both ends of the bone defects. The gelatin p-TCP sponge has osteoconductivity, a pore structure, mechanical strength, and biodegradability, and provides controlled release of growth factors. The gelatin p-TCP sponge with PRP caused favorable bone regeneration in massive bone defects, and may be a clinically applicable option for treating massive bone defects of long bones.