untruthful has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is truthful.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
not expressing or given to expressing the truth
“the statement given under oath was untruthful”
Synonyms, antonyms and the broader and narrower relations for untruthful come from Princeton University’s WordNet. Full sources and licences.