Synonyms for Unbelievable:

adj.

beyond the imagination (adjective)
astonishing, doubtful, dubious, implausible, impossible, improbable, inconceivable, incredible, questionable, staggering, thin, unimaginable, unsubstantial, unthinkable, weak, open to doubt, past belief.
mind-boggling (adjective)
unimaginable.
unreal (adjective)
unsubstantial.

n.

conspicuous, doubtful, dubious, false, implausible, inconceivable, questionable, staggering, ultimate, unadulterated, unimaginable, Unveracious, open to doubt, a bit thick, not to be credited, palpably false, past belief, couldn't be better/worse/nicer etc., like nothing on earth, not...anything better/worse, of your life, with/without distinction. belief, hardly, impossibility, impossible, no-go, possibly, unthinkable, be out of the question, something will not work/start/open etc., with the best will in the world. likely, shaky, thin, unsubstantial, weak, Unconceivable. amazing, astonishing, fantastic, fantastical, good, miraculous, phenomenal, prodigious, stupendous, wonderful, wondrous. unbelievable (noun)
astounding, dumbfounding, dumfounding, fabulous, improbable, incredible, marvellous, marvelous, tall, undreamed, undreamed of, undreamt, undreamt of, unimagined, unlikely.

Other synonyms:

impossibility, Unconceivable. amazing, astonishing, conspicuous, fantastic, stupendous, wondrous. impossible, phenomenal, prodigious, wonderful. fantastical, miraculous. possibly, ultimate. shaky. thin. weak. fantastic
inconceivable.
inconceivable
unimaginable, unthinkable.
incredible
incredible.
monstrous
staggering.
Other relevant words:
amazing, astonishing, doubtful, dubious, fantastic, fantastical, good, implausible, impossibility, impossible, inconceivable, miraculous, phenomenal, possibly, prodigious, questionable, shaky, staggering, stupendous, thin, ultimate, unimaginable, unsubstantial, unthinkable, weak, wonderful, wondrous, Unconceivable, open to doubt, past belief.

Usage examples for unbelievable

  1. She wrote me, soon after that unbelievable mating: I have married Cecil Grimshaw. – O. Henry Memorial Award Prize Stories of 1921 by Various
  2. I prayed for this All night, and yet it's unbelievable – Emblems Of Love by Lascelles Abercrombie
  3. That unbelievable phenomenon had escaped my memory. – The Killer by Stewart Edward White