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)
See also Selyf, a variant of the same name