The keystone flap, an emerging reconstructive option that can be used in many parts of the body, is gaining popularity among reconstructive surgeons. These reliable and versatile flaps can be used for large myelomeningocele closure. A modified bilateral keystone flap was used to achieve tension-free closure of a large thoracolumbar myelomeningocele associated with severe kyphosis in a newborn girl. The flap was modified by undermining in the subfascial plane in the medial aspect of the middle third. This undermining was performed to facilitate flap movement while preserving random musculocutaneous perforators captured within the island of tissues. Laterally, we approached the border of the latissimus dorsi and dissected in the submuscular plane instead of the subfascial plane to preserve more muscular fasciocutaneous perforators. We achieved soft-tissue coverage that was durable, stable, and protective. Wound healing was prompt, and the patient had a satisfactory cosmetic result. No postoperative complications were observed, such as flap necrosis, dehiscence, leakage of cerebrospinal fluid, or infection. The proposed modified keystone flap is a promising addition to the armament of reconstructive surgeons that might improve outcomes and minimize complications in myelomeningocele repair. Keystone flaps provide an ideal reconstructive option for large thoracolumbar myelomeningocele repair. They are reliable, robust, and aesthetically acceptable.