Transverse maxillary deficiency is relatively prevalent in orthodontic patients. This article reports the use of a Miniscrew-Assisted Rapid Palatal Expansion (MARPE) device for the correction of transverse skeletal maxillary constriction in an adult patient. The patient was a 20-year-old girl with a transverse maxillary deficiency and bilateral posterior crossbite. A MARPE device was positioned on her palate using 4 miniscrews. The activation protocol was one-quarter turn per day every other day, with a total activation duration of 10 weeks following with a 3-month retention period. Pre-MARPE and post-MARPE cone-beam computed tomography cross sections presented a skeletal expansion of maxilla. This report confirms the effective treatment of a mature patient with severe maxillary constriction, highly deep palate, and posterior crossbite using a custom-made modification of MARPE.