Seethawaka Fort in Avissawella, Sri Lanka, was the stronghold of the ancient Sitawaka Kingdom, established in the 16th century by King Mayadunne and later expanded by King Rajasinha I. Known for fiercely resisting Portuguese colonial forces, the fort was adjacent to the royal palace and equipped with cannons guarding the riverbank.

Though the fort and palace were destroyed during colonial conflicts, ruins remain near the Avissawella-Panawala road, including foundations and stone platforms within a moat. The site is historically significant as a center of Sinhalese fortress architecture and a symbol of resistance against European domination, now preserved as a protected archaeological monument.