The Collaborative International Dictionary of English v.0.48:

Artless \Art"less\, a.
   1. Wanting art, knowledge, or skill; ignorant; unskillful.
      [1913 Webster]

            Artless of stars and of the moving sand. --Dryden.
      [1913 Webster]

   2. Contrived without skill or art; inartistic. [R.]
      [1913 Webster]

            Artless and massy pillars.            --T. Warton.
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   3. Free from guile, art, craft, or stratagem; characterized
      by simplicity and sincerity; sincere; guileless;
      ingenuous; honest; as, an artless mind; an artless tale.
      [1913 Webster]

            They were plain, artless men, without the least
            appearance of enthusiasm or credulity about them.
                                                  --Porteus.
      [1913 Webster]

            O, how unlike the complex works of man,
            Heaven's easy, artless, unencumbered plan! --Cowper.
      [1913 Webster]

   Syn: Simple; unaffected; sincere; undesigning; guileless;
        unsophisticated; open; frank; candid.
        [1913 Webster]

WordNet (r) 3.0 (2006):

artless
    adj 1: characterized by an inability to mask your feelings; not
           devious; "an ingenuous admission of responsibility" [syn:
           ingenuous, artless] [ant: artful, disingenuous]
    2: simple and natural; without cunning or deceit; "an artless
       manner"; "artless elegance" [ant: artful]
    3: showing lack of art; "an artless translation"
    4: (of persons) lacking art or knowledge [syn: artless,
       uncultivated, uncultured]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "artless":
   aboveboard, arty, awkward, befoolable, big, bluff, blunt,
   born yesterday, broad, brusque, bungling, candid, childlike,
   clumsy, confiding, crude, direct, downright, explicit, foolable,
   forthright, frank, frankhearted, free, free-speaking, free-spoken,
   free-tongued, genuine, green, guileless, gullible, half-assed,
   heart-to-heart, high-sounding, honest, humble, imposing,
   in the raw, inadept, inapt, inartificial, inattentive, incompetent,
   inefficient, inept, inexperienced, inexpert, ingenu, ingenuous,
   innocent, mediocre, naive, native, natural, on the level, open,
   openhearted, ordinary, outspoken, overblown, pedestrian, plain,
   plain-spoken, poor, primitive, pristine, relaxed, round, simple,
   simplehearted, simpleminded, sincere, single-hearted,
   single-minded, skill-less, straight, straight-out, straightforward,
   thoughtless, transparent, true, trustful, trusting, unaffected,
   unapt, unartificial, unassuming, unchecked, uncomplicated,
   unconstrained, undeceptive, undeft, undexterous, undextrous,
   unequivocal, unfacile, unguarded, unintelligent, unpretentious,
   unproficient, unreserved, unrestrained, unschooled, unskilled,
   unskillful, unsophisticated, unstudied, unsullied, unsuspicious,
   untalented, untouched, unwary, virgin, virginal