Modern archosaurs have extensive pneumatic diverticula originating from paranasal and tympanic sinuses. This complex anatomy is present in many fossil archosaurs, but few descriptions of the complete cranial pneumatic system exist. The cranial pneumatic morphology of birds and non-avian theropods are the best studied, but complete description of this anatomy for an ornithomimid was lacking. We describe the cranial pneumaticity of a well-preserved ornithomimid theropod dinosaur, Ornithomimus edmontonicus (RTMP 95.110.1), from computed tomography (CT) scan data and computer-aided reconstruction with quantitative measurements. New details for ornithomimids include an internal promaxillary recess, an internal palatine recess, pneumatic communications between the supraoccipital recess and posterior tympanic recess, and a complex basioccipital recess that connects to the anterior tympanic recess and median pharyngeal system. The pneumatic morphology of the quadrate is similar to modern birds, which have a complete siphoneal duct. We interpret the jugal diverticulum of non-avian theropods to be homologous or at least morphologically related to the avian suborbital diverticulum. Cranial pneumatic morphologies present in Ornithomimus and tyrannosaurids include an extensive internal palatine recesses and a pneumatic articular that is likely derived from a separate siphoneal diverticulum similar to modern birds. Recent phylogenetic hypotheses of theropods imply two equally most parsimonious solutions: that these shared morphologies are either independently derived in each taxon or diagnose Coelurosauria.