Orbital subperiosteal abscess (OSPA) secondary to paranasal sinus mucocele (PSM) is rare, and it may be misdiagnosed as PSM with orbital invasion or even as a malignant neoplasm. The present study explored the computed tomography (CT) and magnetic resonance imaging (MRI) features of OSPA. The cases of 13 patients with OSPA secondary to PSM were retrospectively reviewed. CT had been performed in 12, MRI in 7, and postcontrast MRI in 4. OSPA was revealed as a well-demarcated, spindle-shaped mass that was broad-based and located beneath the superior orbital wall (orbital roof) in 11 and at the medial wall in 2. PSM appeared as an expansile cystic lesion in the ethmofrontal sinus in 7, frontal sinus in 5, and ethmoidal sinus in 1. Because the OSPA was connected to the PSM, it looked like a single lesion involving both the orbit and the sinus. All 12 OSPAs examined on CT were low-density; 9 of the 12 PSMs were low-density and 3 were iso-density. Densities of the OSPAs and PSMs were equal in 4 and slightly different in 8. Five of the 10 OSPAs occurring beneath the orbital roof had unclear boundaries with the PSMs on CT. On MRI, although both OSPAs and PSMs mainly demonstrated hypointensity on T1-weighted images and hyperintensity on T2-weighted images, the signal intensities were slightly different, and linear-shaped hypointensity could be found between them. Postcontrast MRI revealed arch- and ring-shaped enhancement, respectively, at the edge of the OSPA and the PSM. Septal enhancement separated them more clearly. PSM is an important cause of OSPA in adults. CT and MRI can accurately display these entities' characteristic findings and their anatomic relationship, as well as playing an important role in the differential diagnosis.