The Collaborative International Dictionary of English v.0.48:

Arthurian \Ar*thu"ri*an\, a.
   Of or pertaining to King Arthur or his knights. --J. R.
   Symonds.
   [1913 Webster]

         In magnitude, in interest, and as a literary origin,
         the Arthurian invention dwarfs all other things in the
         book.                                    --Saintsbury.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Arthurian
    adj 1: of or relating to King Arthur and the Knights of the
           Round Table