Sialolithiasis is characterized by the obstruction of salivary gland secretion by a calculus. It is associated with swelling, pain, and infection of the affected gland. More than 80% of all salivary calculi occur in the submandibular gland. One reason for this is the makeup of the saliva in the submandibular gland, which includes a higher mucus content, a greater degree of alkalinity, and greater concentrations of calcium and phosphate salts compared with the saliva of the parotid and sublingual glands. Other factors are that its duct is longer and its saliva flows against gravity. Sialoliths that reach several centimeters in diameter (megaliths, or giant calculi) are rare. Perforation of the floor of the mouth by a giant calculus is extremely rare. We report such a case in a 56-year-old man who presented with a 2-day history of severe pain in the left sublingual area and painful swelling in the left submandibular area. Removal of the stone and the left submandibular gland was performed via an extraoral incision. On gross examination, the sialolith measured 5.6 cm.