A mouth cyst is a thin, fluid-filled sac on the inside of your mouth. Also called a mucous cyst or mucocele, the sac is harmless and painless, says the National Institutes of Health (NIH). It may annoy you though, because you feel a bump inside your mouth.
These cysts commonly occur on the inside of the lips but can also form on your tongue, palate, inside of the cheeks, floor of the mouth or around piercings on the tongue or lips. A cyst on the floor of your mouth is called a ranula, and a cyst on the gum is called an epulis. The mucocele sac is bluish and clear and contains clear fluid. Your dentist can usually diagnose a mucous cyst just by looking at it.