| Llywelyn ap Seisyll | |
|---|---|
| King of Gwynedd | |
| Reign | 1018–1023 |
| Predecessor | Aeddan ap Blegywryd |
| Successor | Iago ab Idwal ap Meurig |
| Died | 1023 |
| Spouse | Angharad ferch Maredudd |
| Issue | Gruffydd ap Llywelyn |
| Father | Seisyll |
Llywelyn ap Seisyll (died 1023) was a king of Gwynedd in the 11th century who ruled over the Welsh kingdoms of Gwynedd, Powys and Deheubarth.[1]