What is King Arthur's sirname?

Answer 1

Arthur Pendragon
Surnames as we know them today only begin to appear during the middle ages; during more ancient times a man didn't always need more than one name (societies were much smaller then).

If a man needed an extra name he might take it from a clan he belonged to (Julius Caesar's real name was Gaius, Julius was his clan, and Caesar one branch of that clan) or from some special achievement of his own, or his family (Gaius Marcius became Gaius Marcius Coriolanus after spectacular bravery in the siege of Corioli. His descendants were allowed to retain the name).

King Arthur would not normally have needed a second name (everybody knew who he was), but if he wanted one he might have called himself

Arthur son of Uther

or

Arthur son of Uther son of Constantine

using the names of his father and grandfather.

In Welsh this would be

Arthur ab Uther

Arthur ab Uther ab Cystennin

But Arthur was also the Paramount King of Britain. Arthur's father had called himself Uther Pendragon (Uther the Paramount King), Arthur could also have used the name Arthur Pendragon, if he had wanted to.
Pendragon

Answer 2

According to legend his name was "Arthur ab Uther Pendragon". His name means Arthur, son of Uther, the Head Dragon.

Answer 3

King Arthur's surname is Pendragon. Arthur Pendragon.

Answer 4

pendragon