Resection of the midzone of the continuous epiphysis of a delta phalanx (physiolysis) and its replacement by a free-fat graft from local tissue was performed to improve severe radial angulation of the both thumbs in a 22-month-old child with Rubinstein-Taybi syndrome. Fifty-two months after surgery, satisfactory results were obtained for pinch function, appearance, and radiographic findings. Previous reports have indicated that corrective osteotomy of the thumbs in Rubinstein-Taybi syndrome has an associated risk of incomplete correction and >30% of recurrence, because severe preoperative deformities of the delta phalanx make angulatory osteotomies difficult. Additionally, physiolysis is a less invasive procedure than corrective osteotomy and offers a reduced risk of incomplete or excess correction, physeal injury, or osteonecrosis in younger children. Therefore, physiolysis appears to be useful as an initial means of correcting severe thumb angulations secondary to a delta phalanx. We reported a case in which the physiolysis of the delta phalanx significantly improved severe angular deformities of the thumbs in association with Rubinstein-Taybi syndrome.