Primary intraosseous squamous cell carcinoma (PIOSCC) is a rare lesion that almost exclusively occurs in the jaws. Most PIOSCCs originate from epithelial lining of odontogenic cysts, especially from radicular, residual, and dentigerous cysts. A few cases have been shown to arise from keratocystic odontogenic tumors (KCOTs). This is a report of a case of PIOSCC that arose from an untreated keratocystic odontogenic tumor within a period of less than 2 years of first diagnosis. Upon malignant transformation, the tumor changed from a cystic to a solid pattern and acquired an infiltrative growth pattern, invading and destroying all surrounding bone. This case underscores the importance of early diagnosis and treatment of KCOTs and raises the question whether some of the PIOSCCs classified as "solid type" could, in fact, be late-presenting PIOSCCs arising from KCOTs and other cystic pathologic entities within the jaws.