The thymus is the seat of a diverse array of pathologic conditions given its embryologic roots. Multilocular thymic cysts, although well described, are uncommon, and one associated with rhabdomyomatous elements has not been described previously. A 15-year-old boy complained of sudden-onset chest pain of a month's duration, but was otherwise well. Chest radiographs localized the mass to the anterior mediastinum, arising from the thymus. A computed tomography scan demonstrated the lesion to be a multilocular fluid-containing cyst. A large, 15-cm cyst contiguous with the thymus was removed. Histologic evaluation confirmed a multilocular cyst lined mainly by mucinous epithelium with focal areas of ciliated and squamous lining. A prominent finding was skeletal muscle elements in the form of spider cells, strap-like cells, and foci reminiscent of fetal-type muscle with cross-striations. At the periphery of the cyst, thymic tissue with branching ducts and Hassall corpuscles were noted. No evidence of skin and/or its appendage structures, cartilage, or other differentiated tissue was seen despite generous sampling of the specimen. The muscle elements, most likely, were derived from thymic myoid cells, while the multilocular cyst arose from remnants of the thymomedullary system.