Synonyms for Unbelievable:

adj.

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

n.

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

Other synonyms:

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

Usage examples for unbelievable

  1. I prayed for this All night, and yet it's unbelievable – Emblems Of Love by Lascelles Abercrombie
  2. That unbelievable phenomenon had escaped my memory. – The Killer by Stewart Edward White