Seisyll

Seisyll or Seisyllt is a Welsh male given name. It may refer to:


 * Kings Sisillius I and II, legendary kings of the Britons
 * Seisyll ap Clydog (late 7th century), a king of Ceredigion
 * Seisyll ap Rhun (10th century)
 * Seisyll ap Ednywain or Owain, father of King Llywelyn of Gwynedd
 * Seisyll ap Dyfnwal (12th century), a lord of Upper Gwent
 * Seisyll Bryffwrch (12th century), a poet
 * Seisyll ap Rhun Fychan (13th century)