Llywelyn ap Seisyll
| Llywelyn ap Seisyll | |
|---|---|
| King of Powys and Deheubarth | |
| Reign | 999–1023 | 
| Predecessor | Maredudd ab Owain | 
| Successor | Rhydderch ab Iestyn | 
| 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.