Among all odontogenic cysts, Radicular cyst is the most common inflammatory odontogenic cyst. They develop from epithelial rests of malassez in the PDL, as a result of inflammation. It is a true cyst that occurs in association with the root of the non-vital tooth. An inflammatory response occurs in the periapical tissue, leading to resorption of bone and formation of granulation tissue which are infiltrated by inflammatory cells. Though treatment of choice for smaller cystic lesions involves endodontic therapy, massive cystic lesions don't heal completely with non-surgical endodontic treatment. Massive cystic lesions need Endodontic treatment followed by surgical decompression treatment. In this case report, we present the diagnosis and surgical management of an infected radicular cyst of the mandible.