The Collaborative International Dictionary of English v.0.48:

Rightful \Right"ful\, a.
   1. Righteous; upright; just; good; -- said of persons. [Obs.]
      --Chaucer.
      [1913 Webster]

   2. Consonant to justice; just; as, a rightful cause.
      [1913 Webster]

   3. Having the right or just claim according to established
      laws; being or holding by right; as, the rightful heir to
      a throne or an estate; a rightful king.
      [1913 Webster]

   4. Belonging, held, or possessed by right, or by just claim;
      as, a rightful inheritance; rightful authority.
      [1913 Webster]

   Syn: Just; lawful; true; honest; equitable; proper.
        [1913 Webster]

WordNet (r) 3.0 (2006):

rightful
    adj 1: legally valid; "a rightful inheritance"
    2: having a legally established claim; "the legitimate heir";
       "the true and lawful king" [syn: true(a), lawful,
       rightful(a)]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "rightful":
   according to Hoyle, actionable, applicable, appropriate, apt,
   authentic, authorized, balanced, becoming, befitting, bona fide,
   candid, card-carrying, competent, condign, constitutional, correct,
   de jure, decent, decorous, defensible, deserved, dinkum, due,
   equitable, even, evenhanded, fair, fair and square, fit, fitting,
   following the letter, genuine, good, honest, honest-to-God,
   impartial, inartificial, judicial, juridical, just, justiciable,
   justifiable, justified, kosher, lawful, lawmaking, legal,
   legislative, legit, legitimate, level, licit, lifelike, literal,
   meet, meet and right, merited, natural, naturalistic, nice, normal,
   normative, original, proper, pure, real, realistic, requisite,
   right, right and proper, righteous, sanctioned, seemly, simon-pure,
   simple, sincere, square, statutory, sterling, suitable,
   sure-enough, true, true to life, true to nature, true to reality,
   unadulterated, unaffected, unassumed, unassuming, uncolored,
   unconcocted, uncopied, uncounterfeited, undisguised, undisguising,
   undistorted, unexaggerated, unfabricated, unfanciful, unfeigned,
   unfeigning, unfictitious, unflattering, unimagined, unimitated,
   uninvented, unpretended, unpretending, unqualified, unromantic,
   unsimulated, unspecious, unsynthetic, unvarnished, valid, verbal,
   verbatim, veridical, verisimilar, warrantable, warranted,
   within the law, word-for-word